Fix for gettext rake tasks

These commits are fixing issues with gettext rake tasks, especially
* rake gettext:find
* rake gettext:add_language[XX]
*(where XX is the ISO 639-1 2-letter code for the language)*

To run those rake tasks, you need to define in your Gemfile.dev.rb:
```ruby
  gem "gettext",  "~>2.0",  :require => false
  gem "locale",   "=2.0.9"
  gem 'ruby_parser',        :require => false
```
Still not working
* rake gettext:store_model_attributes

@jzigmund does this pull request require this change?  While it might be worthwhile to investigate, it doesn't seem related to the subject of the PR.

I'm not sure how to detect who's adding it already other than grepping the code base or overriding $LOAD_PATH << , +, etc. methods. (or using Module#prepend on 2.0)