Group require statements together.
Remove unnecessary module definition.
Tabnav depends on ActionPack, so require it from tabnav.rb
Test tabnav with the latest production version of Rails.

Rails had to be added as a development dependency because rspec-rails loads elements that depend on the whole Rails framework to be loaded in.

Technically though, the runtime dependency is only ActionPack.
Separate logical blocks.
Regenerate Rspec.