Validate JS on .js file changes only

We were triggering a JS validation when .json files were changed.

![screen shot 2014-03-13 at 14 10 54](https://f.cloud.github.com/assets/85783/2410287/ab5abf1c-aabb-11e3-9b62-e1ecce045af9.PNG)


![ ](http://media.giphy.com/media/9Sxp3YOKKFEBi/giphy.gif)