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.
Rename RAILS_VERSION to RAILS_BRANCH