Added missing postcss-clean dev dependency

Author: matthew-dean

package.json

@@ -25,7 +25,8 @@
     "gulp": "^3.9.0",
     "gulp-mocha": "^2.2.0",
     "less": "^2.7.1",
-    "postcss": "^5.0.10"
+    "postcss": "^5.0.10",
+    "postcss-clean": "^1.0.2"
   },
   "scripts": {
     "test": "gulp test"