Remove and ignore .ruby-version

* Unfortunately this file doesn't play nice across different Ruby
  version managers. I tend to use the format "2.0" as a generalized way
  for RVM to automatically pick up the most current Ruby version.
  Apparently rbenv can't do that. Let's just kill it and have our
  Gemfile do the talking. We can create and manage our own as
  wanted/needed.

[Closes #12]