Clarify Rails version support

Please also remove the version 3.2 from the GitHub project settings. It should just say "Tolk is a web interface for doing i18n translations packaged as an engine for Rails applications" or even the shorter "Tolk is a web interface for doing i18n translations, packaged as a Rails engine".

This PR fixes #72.

Note that the Ruby requirement in the gemspec will eventually need to be loosened for Rails 4.1. (Probably to `s.add_runtime_dependency 'rails', '>= 4.0', '< 4.2'`.)