Updates mapbox.js to 1.4.2 and adds rake task for building

The biggest change is the addition of the `build_source` rake task. To run this, first set the ENV variable `MAPBOX_JS_PATH` to your local copy of mapbox.js. When `build_source` is run, it will build all mapbox.js files and copy them into this gem's directory. Hopefully this makes it easier to update with mapbox.js in the future.

Also, it appears that Mapbox's IE specific css file has been removed, so I deleted it from this repository as well. This is a breaking change for some users, and it's of course up to you if you wish to keep this file out of the gem.

Thanks!