Add depend_on_asset for all fonts to make rails asset pipeline happy.

It is now mandatory to add `depend_on_asset` on assets that reference other assets to make rails asset pipeline happy, as described in [here](https://github.com/rails/sprockets-rails/pull/96). 

I thought that it was only mandatory for `erb` assets, but my sprockets complained that `depend_on_asset "fontawesome-webfont.eot` should be added to `font-awesome.scss`, and when I did, it stopped complaining. I think that this is because `scss` files must be precompiled anyway, so `sprockets` picks this up.

This gem is not usable in rails if these lines aren't added. When I updated `sprockets` I detected these problems in your gem and another, so please see [this discussion](https://github.com/joliss/jquery-ui-rails/pull/59) for further reference.

I tested this with the following gem versions:

```ruby
gem 'rails', '3.2.16'
ruby '2.0.0'
gem 'sprockets', '2.2.2.backport2'
gem 'sprockets-rails', '2.0.0.backport1'
```

Should work for `rails 4` as well, although I didn't test it.

Cheers and Merry Xmas :)