add missing 'cache' import in routes/skins

lib/routes/skins.js

@@ -1,6 +1,7 @@
 var logging = require("../logging");
 var helpers = require("../helpers");
 var skins = require("../skins");
+var cache = require("../cache");
 var path = require("path");
 var lwip = require("lwip");
 var url = require("url");