Compress text/javascript files, too

This was preventing large files like `bundled.js` from being compressed over the wire. They were being sent with `Content-type: text/javascript` instead of `Content-type: application/javascript`.

As for what they _should_ be sent with, it seems [murky](http://stackoverflow.com/a/4101763/388951).

The `COMPRESS_DEBUG` param enables gzip for the local Flask dev server. Seems very 12 factory.