Downgrade clean-css, image URLs were wrong.

package.json

@@ -37,7 +37,7 @@
     "backbone.browserStorage": "0.0.3",
     "backbone.overview": "0.0.3",
     "bower": "latest",
-    "clean-css-cli": "^4.0.0",
+    "clean-css": "^3.4.19",
     "crypto-js": "3.1.2-5",
     "greenkeeper": "^4.1.0",
     "grunt": "^1.0.1",