Install bootstrap and font-awesome from gems

Often, it's much easier to update assets such as Bootstrap and Font-Awesome
when they are installed from gems rather than source. Install bootstrap from
anjlab/bootstrap-rails and Font Awesome from bokmann/font-awesome-rails.

I'm guessing you installed these assets from source for a reason, but I figured
I'd submit a PR anyway.

Signed-off-by: David Celis <me@davidcel.is>