Use uniform .ruby-version file

* Add .ruby-version (supported by all major version managers).
* Remove .rbenv-version (rbenv checks .ruby-version first).
* Update .rvmrc:
  * Create .rvmrc via RVM's command.
  * Remove extra commented lines from .rvmrc.
  * Setup .rvmrc to pull ruby version from .ruby-version file.
Remove .rvmrc

Gemset support is not as crticical as it used to be. Remove .rvmrc as RVM
uses .ruby-version by default if no .rvmrc is present.
Merge pull request #398 from stevenhaddox/ruby-version-updates

Use uniform .ruby-version file