Require assets with logical paths, where possible

The latest versions of Sprockets parse `bower.json` and set-up the
logical path to be translated to the source file specified in the
'main' property.

N.B.
- the path for the main file of older versions of angular-ui-date is
  incorrect.
  See https://github.com/angular-ui/ui-date/commit/346e1e0126ec9f6fbe0e7f717a6d911d6c21525b.
- this is not yet working with Caret.js.
- 2.2.2.backport2 had broken support for Bower projects.
  See 960e2f5.

Signed-off-by: Alex Coles <alex@alexbcoles.com>