Remove unused/duplicate require

We use **RouteTranslator** in quite a few projects, but not all of them depend on the **ActionMailer** gem.

Is there any reason, why RouteTranslator explicitly requires *action_mailer*? After refactoring the ```require``` section, anything seems to work smoothly.

Before:
```ruby
require 'active_support'
require 'action_controller'
require 'action_mailer'
require 'action_dispatch'
```

After:
```ruby
require 'active_support'
```
The *action_controller* and *action_dispatch* are required again when needed (extensions/{route_set.rb, mapper.rb, action_controller.rb}).