Add PHPCS as a dev dependency.

composer.json

@@ -16,7 +16,8 @@
         "symfony/event-dispatcher": "~2.1"
     },
     "require-dev": {
-        "phpunit/phpunit": "~3.7"
+        "phpunit/phpunit": "~3.7",
+        "squizlabs/php_codesniffer": "~1.4"
     },
     "autoload": {
         "psr-0": { "Solarium\\": "library/" }