Raise on missing translations in development/test

Lessons learned from http://robots.thoughtbot.com/foolproof-i18n-setup-in-rails
Could this be generalized to a `uncomment_in_file` method? I think that might be easier to read:

``` ruby
uncomment_in_file "config/environments/#{environment}.rb",
  'config.action_view.raise_on_missing_translations = true'
```