Add RVM workflow files `.ruby-version` and `.ruby-gemset`

These files make RVM automatically change to the ruby version and
gemset selected, conveniently automating a part of the typical
development workflow when switching between projects.

See https://rvm.io/workflow/projects
Merge pull request #9 from ms-ati/add-rvm-files

Add RVM workflow files `.ruby-version` and `.ruby-gemset`