🔧 QA: enable feature completeness check for the Debug ruleset

This:
* Adds convenience scripts to the `composer.json` file to check the feature completeness of sniffs in this repo.
* And adds running of this script to the Travis build.

Author: jrfnl

.travis.yml

@@ -59,6 +59,11 @@ script:
   # Lint PHP files against parse errors.
   - composer lint
 
+  # Check that any sniffs available are feature complete.
+  # This also acts as an integration test for the feature completeness script,
+  # which is why it is run against various PHP versions and not in the "Sniff" stage.
+  - composer check-complete
+
   # Validate the composer.json file on low/high PHP versions.
   # @link https://getcomposer.org/doc/03-cli.md#validate
   - composer validate --no-check-all --strict

composer.json

@@ -42,6 +42,9 @@
         ],
         "fix-cs": [
             "@php ./vendor/squizlabs/php_codesniffer/bin/phpcbf"
+        ],
+        "check-complete": [
+            "@php ./bin/phpcs-check-feature-completeness ./Debug"
         ]
     }
 }