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 to generate the required_ruby_version for
 suspenders and the ruby call for the generated Gemfile

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