Remove Ruby Versioning Files 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.

I've also set the gem's minimum ruby version to 1.9.3.  Although this is likely covered by the requirement for rails 4+, it makes sense to make the requirement explicit.