Travis: ignore PHP deprecation notices for stable PHPCS releases

The unit tests will fail when a PHP warning/notice/deprecation notice is encountered.

Deprecation notices thrown by already released PHPCS versions won't get fixed anymore (in that version), so failing the unit tests on those is moot and will skew the reliability of the Travis results.

.travis.yml

@@ -150,6 +150,13 @@ before_install:
       phpenv config-rm xdebug.ini || echo 'No xdebug config.'
     fi
 
+  # 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 [[ "$TRAVIS_BUILD_STAGE_NAME" != "Sniff" && $PHPCS_VERSION != "dev-master" ]]; then
+      echo 'error_reporting = E_ALL & ~E_DEPRECATED' >> ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/travis.ini
+    fi
+
   # Allow for testing with different PHP ini configurations.
   - |
     if [[ "$CUSTOM_INI" == 1 ]]; then