travis: Fail on deprecation warning

This commit makes Travis fail on deprecation warnings for new versions
of PHP and ensures that checks are made against all versions from PHP
5.6 onwards

.travis.yml

@@ -19,7 +19,7 @@ script:
   - ~/.config/composer/vendor/bin/phpcs . --standard=phpcs.xml --warning-severity=0 --extensions=php -p;
   # Check PHP compatibility for the lowest supported version
   - if [[ $TRAVIS_PHP_VERSION == "5.6" ]]; then
-      ~/.config/composer/vendor/bin/phpcs . --standard=phpcompatibility.xml --warning-severity=0 --extensions=php -p;
+      ~/.config/composer/vendor/bin/phpcs . --standard=phpcompatibility.xml --extensions=php -p;
     fi
   # Run unit tests (stable)
   - if [[ $TRAVIS_PHP_VERSION == "7.0" ]]; then
@@ -29,7 +29,7 @@ script:
   # Check PHP compatibility for all versions, starting at the lowest supported version in order to detect breaking changes
   - if [[ $TRAVIS_PHP_VERSION == "nightly" ]]; then
       phpunit --configuration=phpunit.xml --include-path=lib/;
-      ~/.config/composer/vendor/bin/phpcs . --standard=PHPCompatibility --warning-severity=0 --extensions=php -p --runtime-set testVersion 5.6-;
+      ~/.config/composer/vendor/bin/phpcs . --standard=PHPCompatibility --extensions=php -p --runtime-set testVersion 5.6-;
     fi
 
 matrix:

phpcompatibility.xml

@@ -8,7 +8,7 @@
   <!--
 
   -->
-  <config name="testVersion" value="5.6"/>
+  <config name="testVersion" value="5.6-"/>
   <rule ref="PHPCompatibility">
     <!--
     "PHP 7 changes how most errors are reported by PHP. Instead of reporting