Remove versioning from Cardboard

Developers should probably work against the most recent version of ruby. And we
should have automated tests to check that the gem works for all suported ruby
versions. The ".ruby-version" and ".ruby-gemset" files have been added to
.gitignore so developers can set those up individually.
Merge pull request #77 from joeyjoejoejr/remove_ruby_version

Remove Ruby Versioning Files From Cardboard