Add haml-rails dependency

If a Rails application that uses Spree with your extension is not using any HAML engine, an ActionView::Template error will be raised:

```
ActionView::MissingTemplate in Spree::Checkout#edit

Missing partial spree/checkout/terms_and_conditions_checkbox with {:locale=>[:en], :formats=>[:html], :handlers=>[:erb, :builder, :raw, :ruby, :jbuilder, :coffee, :rabl], :versions=>[:v1]}. Searched in:
  * "/Users/Demo/Work/datenspiel/stoffekontor-test/spree_test/app/views"
  * "/Users/Demo/.rvm/gems/ruby-2.0.0-p247@sk/gems/spree_terms_and_conditions-2.1.4/app/views"
  * "/Users/Demo/.rvm/gems/ruby-2.0.0-p247@sk/bundler/gems/spree_auth_devise-92b424f05c9f/app/views"
  * "/Users/Demo/.rvm/gems/ruby-2.0.0-p247@sk/gems/devise-3.2.2/app/views"
  * "/Users/Demo/.rvm/gems/ruby-2.0.0-p247@sk/bundler/gems/spree-3bb439fb45f5/frontend/app/views"
  * "/Users/Demo/.rvm/gems/ruby-2.0.0-p247@sk/bundler/gems/spree-3bb439fb45f5/backend/app/views"
  * "/Users/Demo/.rvm/gems/ruby-2.0.0-p247@sk/bundler/gems/spree-3bb439fb45f5/api/app/views"
  * "/Users/Demo/.rvm/gems/ruby-2.0.0-p247@sk/bundler/gems/spree-3bb439fb45f5/core/app/views"
  * "/Users/Demo/.rvm/gems/ruby-2.0.0-p247@sk/gems/kaminari-0.15.1/app/views"

```

I added an explicit ```haml-rails``` dependency to the gemspec and some tests.