Test against different Rails version on Travis

`ENV["RAILS_VERSION"]` was being set to Rails branch names (like
`4-0-stable`), but was being used as a normal version constraint. This led to
declarations like:

    gem "rails", "~> 4-0-stable.0"
   # or
   gem "rails", "~> 4-1-stable.0"

Both of these were translated by Rubygems into "get the latest version", so
they both tested against the same version of Rails, currently 4.1.2.rc1.

You can see this here:
https://travis-ci.org/thoughtbot/griddler/builds/26420309

This commit makes it so we always interpret `ENV["RAILS_VERSION"]` as a branch 
name. We still fall back to testing against Rails 4.0.0 if the environment 
variable is unset.