Updated package.json

Changed usemin to a newer version.
Added concat because usemin requires it.

This was done at the same time as using usemin
when grunting was fixed.
Fixed usemin and rev tasks in the Gruntfile

Now usemin concats and uglifies the scripts correctly when
running `grunt build`.

Removed revving of images and fonts because it seems like
they don't work correctly with Crafty at the moment.