Fix ~/.npm permissions

When trying to run `gulp install [xxx] --save-dev` without using sudo, it will fail because of permission issues with ~/.npm. As highlighted on [stackoverflow](http://stackoverflow.com/questions/12231846/npm-will-not-install-express) the solution is to chmod the ~/.npm directory to the vagrant user.

I attempted a solution in the script, but am unsure how to test it, and wanted to submit this anyway. Perhaps someone can pick this up and create a functioning pull request, verify that this works, or recommend how I go about testing it. :)

Thanks!