Add Rails::VERSION module to test monkey-patches

rspec-rails (or more specifically RailsExampleGroup) expects the
new Rails::VERSION module to exist. We could restrict the rspec-rails
version but that's probably asking for more problems in the long
run.

Before this commit, tests using rspec-rails 2.13.x failed with:

/gems/rspec-rails-2.13.2/lib/rspec/rails/example/rails_example_group.rb:10:in
`<module:RailsExampleGroup>': uninitialized constant Rails::VERSION (NameError)
Merge pull request #28 from injekt/rails-version-module

Add Rails::VERSION module to test monkey-patches