Use sprockets-rails 2.2.x on base app and allow greater version

At the moment, this Gemfile wont work:
```ruby
gem "rails", github: "rails/rails"
gem 'sprockets-rails', github: 'rails/sprockets-rails'
gem 'sass-rails', github: "rails/sass-rails"
```

This wont work because:
`sass-rails`(master) requires `sprockets` < 3.0 (https://github.com/rails/sass-rails/blob/63818c118e10258d760fa6b82cf9bedcb0a6c9cb/sass-rails.gemspec#L21)
`sprockets-rails`(master) requires `sprockets` 3.0.x (https://github.com/rails/sprockets-rails/blob/059d4707d27681a14cdcbbcb42a984f811c6dae5/sprockets-rails.gemspec#L14)


sprockets-rails 2.2.1 has been released, http://rubygems.org/gems/sprockets-rails/versions/2.2.1, so we can make rails requires version 2.2+, and have the default app generator pointing to `~> 2.2.1`.

This will fix the broken test `test_generation_runs_bundle_install_with_full_and_mountable`

@rafaelfranca thoughts? lemme know if something in here doesnt make sense.
Can't we just remove this gemfile entry entirely if `rails` has a hard dep on `sprockets-rails`?