Unify Ruby version for gem and suspended apps

    One Ruby to rule them all, One Ruby to find them,
    One Ruby to bring them all and in the light bind them
    In the Land of .ruby-version where the Fluorescents lie.

When creating new apps, there is little reason to use an older version
of Ruby than the most recent. For most use cases, having the latest
version is going to be the best approach.

* Move Ruby version to .ruby-version for gem
* Read this file into Suspenders::RUBY_VERSION
* Use Suspenders::RUBY_VERSION everywhere we need it, such as the
* Gemfile and generated Travis config.
* Update README documentation about Ruby and Rails versions.
* Add color to Travis output.

This is based on a conversation with @derekprior and @mrgilman.