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 (broken off into #1153)

> Specified version, because 'rake gettext:add_langugage'
and 'rake gettext:find' are working we expect. It didn't work with
the latest version.

@jzigmund what do you mean by not working with the latest version?  Can you be detailed in describing what's failing?  Can you see if the rake task was removed/changed or if anyone has reported the same issue on their repo?