Merge pull request #2695 from guillaumelecerf/fix_cache_gc_with_cachedir_devnull

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

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);