Remove lodash.merge require() hack

See:
https://github.com/callemall/material-ui/pull/4380#issuecomment-25089455
2

My PR to material-ui to replace the full `lodash` with just the needed
functions was merged. So we can remove this hack.

package.json

@@ -29,7 +29,6 @@
     "iso-639-1": "^1.2.1",
     "languagedetect": "^1.1.1",
     "location-history": "^1.0.0",
-    "lodash.merge": "^4.6.0",
     "material-ui": "^0.16.0",
     "mkdirp": "^0.5.1",
     "musicmetadata": "^2.0.2",