Use an RVM gemset

A gem set is a stand-alone group of gems, just for this project. It
keeps a project’s dependencies clean - i.e. the only gems available in
this project are those that this project uses. No leakage.

Usually I use rbenv, which won’t be affected by this. I still wanted to
have RVM set up to use, though, so that it doesn’t matter what version
manager I use.
Add .ruby-version file

Specifies the version of ruby to use. Will be used by both rbenv and
rvm.
Merge branch 'setup-for-rvm'