Updated Travis CI configuration

Run builds over all PHP supported versions with lowest/locked/latest deps.

.travis.yml

@@ -10,32 +10,60 @@ env:
   global:
     - COMPOSER_ARGS="--no-interaction"
     - COVERAGE_DEPS="php-coveralls/php-coveralls"
-    - LEGACY_DEPS="phpunit/phpunit"
 
 matrix:
   include:
     - php: 5.6
+      env:
+        - DEPS=lowest
+    - php: 5.6
+      env:
+        - DEPS=locked
+        - LEGACY_DEPS="phpunit/phpunit"
+    - php: 5.6
+      env:
+        - DEPS=latest
     - php: 7
       env:
-        - CS_CHECK=true
+        - DEPS=lowest
+    - php: 7
+      env:
+        - DEPS=locked
+        - LEGACY_DEPS="phpunit/phpunit"
+    - php: 7
+      env:
+        - DEPS=latest
     - php: 7.1
       env:
+        - DEPS=lowest
+    - php: 7.1
+      env:
+        - DEPS=locked
+        - CS_CHECK=true
         - TEST_COVERAGE=true
+    - php: 7.1
+      env:
+        - DEPS=latest
     - php: 7.2
-  allow_failures:
+      env:
+        - DEPS=lowest
+    - php: 7.2
+      env:
+        - DEPS=locked
     - php: 7.2
+      env:
+        - DEPS=latest
 
 before_install:
   - if [[ $TEST_COVERAGE != 'true' ]]; then phpenv config-rm xdebug.ini || return 0 ; fi
-  - travis_retry composer self-update
 
 install:
   - travis_retry composer install $COMPOSER_ARGS --ignore-platform-reqs
-  - if [[ $TRAVIS_PHP_VERSION =~ ^5.6 ]]; then travis_retry composer update $COMPOSER_ARGS --with-dependencies $LEGACY_DEPS ; fi
+  - if [[ $LEGACY_DEPS != '' ]]; then travis_retry composer update $COMPOSER_ARGS --with-dependencies $LEGACY_DEPS ; fi
+  - if [[ $DEPS == 'latest' ]]; then travis_retry composer update $COMPOSER_ARGS ; fi
+  - if [[ $DEPS == 'lowest' ]]; then travis_retry composer update --prefer-lowest --prefer-stable $COMPOSER_ARGS ; fi
   - if [[ $TEST_COVERAGE == 'true' ]]; then travis_retry composer require --dev $COMPOSER_ARGS $COVERAGE_DEPS ; fi
-  - stty cols 120
-  - export COLUMNS=120
-  - composer show
+  - stty cols 120 && composer show
 
 script:
   - if [[ $TEST_COVERAGE == 'true' ]]; then composer test-coverage ; else composer test ; fi