Fix Cache::gc() when COMPOSER_CACHE_DIR=/dev/null

If we set COMPOSER_CACHE_DIR=/dev/null, and the garbage collector
is triggered, we end up with the following error :

The "/dev/null/" directory does not exist.

This is because the Cache::gc() function does not check for Cache::enabled
and instanciates a Finder unconditionnaly.

Fix this by adding a check on Cache::enabled.

src/Composer/Cache.php

@@ -143,6 +143,8 @@ class Cache
     }
 
     public function gc($ttl, $maxSize)
+    {
+        if ($this->enabled)
         {
             $expire = new \DateTime();
             $expire->modify('-'.$ttl.' seconds');
@@ -168,6 +170,9 @@ class Cache
             return true;
         }
 
+        return false;
+    }
+
     public function sha1($file)
     {
         $file = preg_replace('{[^'.$this->whitelist.']}i', '-', $file);