Travis: run the code style related and ruleset 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.

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.
* The output of the Travis jobs will be a lot easier to decipher as it only shows the output related to that particular stage.

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

@@ -26,16 +26,35 @@ env:
   # Lowest supported PHPCS + WPCS versions.
   - PHPCS_BRANCH="3.3.1" WPCS_BRANCH="2.1.0"
 
-matrix:
+stages:
+  - sniff
+  - test
+
+jobs:
   fast_finish: true
   include:
-    # Seperate builds for PHP 7.2 with additional checks.
-    - php: 7.2
-      env: PHPCS_BRANCH="dev-master" WPCS_BRANCH="2.1.0" LINT=1 SNIFF=1
+    - stage: sniff
+      php: 7.3
+      env: PHPCS_BRANCH="dev-master"
       addons:
         apt:
           packages:
             - libxml2-utils
+      script:
+        # Check the codestyle of the WPThemeReview codebase.
+        - $(pwd)/vendor/bin/phpcs
+
+        # Validate the xml files.
+        # @link http://xmlsoft.org/xmllint.html
+        - xmllint --noout --schema ./vendor/squizlabs/php_codesniffer/phpcs.xsd ./*/ruleset.xml
+
+        # Check the code-style consistency of the xml files.
+        - diff -B --tabsize=4 ./WPThemeReview/ruleset.xml <(xmllint --format "./WPThemeReview/ruleset.xml")
+
+    # Seperate test builds for PHP 7.2 with reversed PHPCS vs WPCS branches.
+    - stage: test
+      php: 7.2
+      env: PHPCS_BRANCH="dev-master" WPCS_BRANCH="2.1.0" LINT=1
     - php: 7.2
       env: PHPCS_BRANCH="3.3.1" WPCS_BRANCH="dev-master"
 
@@ -49,7 +68,7 @@ before_install:
   - export XMLLINT_INDENT="	"
   - composer require squizlabs/php_codesniffer:${PHPCS_BRANCH} wp-coding-standards/wpcs:${WPCS_BRANCH} --no-update --no-suggest --no-scripts
   - |
-    if [[ "$SNIFF" == "1" ]]; then
+    if [[ "$TRAVIS_BUILD_STAGE_NAME" == "Sniff" ]]; then
       composer install --dev --no-suggest
       # The Composer PHPCS plugin takes care of the installed_paths.
     else
@@ -61,20 +80,15 @@ before_install:
 script:
   # Lint the PHP files against parse errors.
   - if [[ "$LINT" == "1" ]]; then if find . -path ./vendor -prune -o -name "*.php" -exec php -l {} \; | grep "^[Parse error|Fatal error]"; then exit 1; fi; fi
+
+  # Validate the composer.json file.
+  # @link https://getcomposer.org/doc/03-cli.md#validate
+  - if [[ "$LINT" == "1" ]]; then composer validate --no-check-all --strict; fi
+
   # Run the unit tests.
   - |
     if [[ ${TRAVIS_PHP_VERSION:0:3} > "7.1" ]]; then
       vendor/bin/phpunit --filter WPThemeReview $(pwd)/vendor/squizlabs/php_codesniffer/tests/AllTests.php
     else
       phpunit --filter WPThemeReview $(pwd)/vendor/squizlabs/php_codesniffer/tests/AllTests.php
     fi
-  # Check the codestyle of the WPThemeReview codebase.
-  - if [[ "$SNIFF" == "1" ]]; then $(pwd)/vendor/bin/phpcs; fi
-  # Validate the xml files.
-  # @link http://xmlsoft.org/xmllint.html
-  - if [[ "$SNIFF" == "1" ]]; then xmllint --noout --schema ./vendor/squizlabs/php_codesniffer/phpcs.xsd ./*/ruleset.xml; fi
-  # Check the code-style consistency of the xml files.
-  - if [[ "$SNIFF" == "1" ]]; then diff -B --tabsize=4 ./WPThemeReview/ruleset.xml <(xmllint --format "./WPThemeReview/ruleset.xml"); fi
-  # Validate the composer.json file.
-  # @link https://getcomposer.org/doc/03-cli.md#validate
-  - if [[ "$LINT" == "1" ]]; then composer validate --no-check-all --strict; fi