Add PHPCompatibility ruleset to coding standards check

composer.json

@@ -19,6 +19,7 @@
     },
     "require-dev": {
         "ext-gd": "*",
+        "phpcompatibility/php-compatibility": "*",
         "phpunit/phpunit": "*",
         "squizlabs/php_codesniffer": "*"
     },

tests/ruleset.xml

@@ -1,5 +1,10 @@
 <?xml version="1.0"?>
 <ruleset name="PSR2Modified">
+    <!-- Run against the PHPCompatibility ruleset. -->
+    <rule ref="PHPCompatibility" />
+    <config name="installed_paths" value="vendor/phpcompatibility/php-compatibility" />
+
+    <!-- Run against the PSR2 ruleset with some exclusions. -->
     <rule ref="PSR2">
         <exclude name="Generic.Functions.FunctionCallArgumentSpacing.TooMuchSpaceAfterComma" />
         <exclude name="PSR1.Methods.CamelCapsMethodName.NotCamelCaps" />