Rails 4.0.0

Upgrades to rails 4.0.0.

The list of changes (that I can remember):

- [x] Updated travis-ci to use ruby 2.0.0
- [x] Updated all gems, using a few from github.
- [x] Implemented strong_parameters, removed all attr_accessible.
- [x] Updated scopes to use `->`.
- [x] Went through the guide at guide.rubyonrails.org and updated all other requirements.
- [x] Removed :assets group and updated Bundler.require.

**TODO:** Fix `stylesheet_link_tag` in `layouts/email.haml`.