Travis: run the code style related checks in separate stages

Travis now offers stages. Using stages we can:
- Run the code style related checks before running any unit tests and stop the build early if any are detected.
- Remove the duplicate unit test runs - i.e. previously we had an extra (third) build against PHP 7.1 (now changed to 7.3) which would run the code style related checks, but would also re-run the unit tests. This extra build will now no longer run the unit tests.
- Add a separate "quicktest" stage for non-PR/merge builds.
    The "quicktest" stage will only run a CS check, ruleset check, linting and the unit tests against low/high PHP/PHPCS/WPCS combinations. This should catch most issues.
    The more comprehensive complete build against a larger combination of PHP/PHPCS/WPCS combination will now only be run on PRs and merges to `develop`/`master`.

While this does mean that the unit tests will run with a slight delay (the `Sniff` stage has to finish before they start), it also means that we:
* Get code style errors reported earlier as it's been moved to be the first stage and the build will just stop if any are found.
* We won't be wasting Travis's resources on builds which will have to be re-run anyway.

Ref: https://docs.travis-ci.com/user/build-stages/

Note that `Allowed failures` is no longer listed as a separate block in the Travis result overview, but is _is_ respected.

For more discussion about this:
* travis-ci/travis-ci#7789
* https://travis-ci.community/t/always-show-allow-failures-allowed-failures-when-build-stages-are-used/217
* https://travis-ci.community/t/work-out-kinks-in-interactions-between-stages-allow-fail-and-fast-finish/1090
* travis-ci/travis-ci#9677

.travis.yml

@@ -1,5 +1,3 @@
-sudo: false
-
 language:
     - php
 
@@ -30,16 +28,55 @@ env:
   - PHPCS_BRANCH="3.4.2" WPCS="dev-develop"
   - PHPCS_BRANCH="3.4.2" WPCS="2.1.1"
 
+# Define the stages used.
+# For non-PRs, only the sniff, ruleset and quicktest stages are run.
+# For pull requests and merges, the full script is run (skipping quicktest).
+# Note: for pull requests, "develop" should be the base branch name.
+# See: https://docs.travis-ci.com/user/conditions-v1
+stages:
+  - name: sniff
+  - name: rulesets
+  - name: quicktest
+    if: type = push AND branch NOT IN (master, develop)
+  - name: test
+    if: branch IN (master, develop)
+
 matrix:
   fast_finish: true
   include:
-    # Extra build to check for XML codestyle.
-    - php: 7.1
-      env: PHPCS_BRANCH="dev-master" WPCS="^2.1.1" SNIFF=1
+    #### SNIFF STAGE ####
+    - stage: sniff
+      php: 7.3
+      env: PHPCS_BRANCH="dev-master" WPCS="dev-master"
       addons:
-          apt:
-              packages:
-                - libxml2-utils
+        apt:
+          packages:
+            - libxml2-utils
+      script:
+        # Check the codestyle of the files within YoastCS.
+        - composer check-cs
+
+        # Validate the xml files.
+        # @link http://xmlsoft.org/xmllint.html
+        # For the build to properly error when validating against a scheme, these each have to be in their own condition.
+        - xmllint --noout --schema ./vendor/squizlabs/php_codesniffer/phpcs.xsd ./Yoast/ruleset.xml
+        - xmllint --noout ./Yoast/Docs/*/*Standard.xml
+
+        # Check the code-style consistency of the xml files.
+        - diff -B --tabsize=4 ./Yoast/ruleset.xml <(xmllint --format "./Yoast/ruleset.xml")
+
+    #### QUICK TEST STAGE ####
+    # This is a much quicker test which only runs the unit tests and linting against low/high
+    # supported PHP/PHPCS/WPCS combinations.
+    - stage: quicktest
+      php: 7.3
+      env: PHPCS_BRANCH="dev-master" WPCS="dev-develop"LINT=1
+    - php: 7.3
+      env: PHPCS_BRANCH="3.3.1" WPCS="2.1.1"
+    - php: 5.4
+      env: PHPCS_BRANCH="dev-master" WPCS="2.1.1" LINT=1
+    - php: 5.4
+      env: PHPCS_BRANCH="3.3.1" WPCS="dev-develop"
 
   allow_failures:
     # Allow failures for unstable builds.
@@ -54,11 +91,9 @@ before_install:
     # On stable PHPCS versions, allow for PHP deprecation notices.
     # Unit tests don't need to fail on those for stable releases where those issues won't get fixed anymore.
     - |
-      if [[ $PHPCS_BRANCH != "dev-master" && $WPCS != "dev-develop" ]]; then
+      if [[ "$TRAVIS_BUILD_STAGE_NAME" != "Sniff" && $PHPCS_BRANCH != "dev-master" && $WPCS != "dev-develop" ]]; then
         echo 'error_reporting = E_ALL & ~E_DEPRECATED' >> ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/travis.ini
       fi
-    - php -r "echo ini_get('error_reporting');"
-
 
     - export XMLLINT_INDENT="	"
     - export PHPCS_DIR=$(pwd)/vendor/squizlabs/php_codesniffer
@@ -68,7 +103,7 @@ before_install:
     # Set the WPCS version to test against.
     - composer require wp-coding-standards/wpcs:${WPCS} --no-update --no-suggest --no-scripts
     - |
-      if [[ "$SNIFF" == "1" ]]; then
+      if [[ "$TRAVIS_BUILD_STAGE_NAME" == "Sniff" ]]; then
           composer install --dev --no-suggest
           # The DealerDirect Composer plugin script takes care of the installed_paths.
       else
@@ -83,21 +118,17 @@ before_install:
     - $PHPCS_BIN -i
 
 script:
+    # Lint the PHP files against parse errors.
     - if [[ "$PHPLINT" == "1" ]]; then if find . -path ./vendor -prune -o -name "*.php" -exec php -l {} \; | grep "^[Parse error|Fatal error]"; then exit 1; fi; fi
+
+    # Run the unit tests.
     - |
       if [[ ${TRAVIS_PHP_VERSION:0:3} > "7.1" ]]; then
         vendor/bin/phpunit --filter Yoast --bootstrap="$PHPCS_DIR/tests/bootstrap.php" $PHPCS_DIR/tests/AllTests.php
       else
         phpunit --filter Yoast --bootstrap="$PHPCS_DIR/tests/bootstrap.php" $PHPCS_DIR/tests/AllTests.php
       fi
-    # Check the codestyle of the files within YoastCS.
-    - if [[ "$SNIFF" == "1" ]]; then composer check-cs; fi
-    # Validate the xml files.
-    # @link http://xmlsoft.org/xmllint.html
-    - if [[ "$SNIFF" == "1" ]]; then xmllint --noout --schema ./vendor/squizlabs/php_codesniffer/phpcs.xsd ./Yoast/ruleset.xml; fi
-    - if [[ "$SNIFF" == "1" ]]; then xmllint --noout ./Yoast/Docs/*/*Standard.xml; fi
-    # Check the code-style consistency of the xml files.
-    - if [[ "$SNIFF" == "1" ]]; then diff -B --tabsize=4 ./Yoast/ruleset.xml <(xmllint --format "./Yoast/ruleset.xml"); fi
+
     # Validate the composer.json file.
     # @link https://getcomposer.org/doc/03-cli.md#validate
     - if [[ "$PHPLINT" == "1" ]]; then composer validate --no-check-all --strict; fi