Replacing `.rvmrc` with `.ruby-gemset`

In order to avoid duplicating the Ruby version between the `.rvmrc`
and the `.ruby-version`, we'll get rid of the `.rvmrc` and use the
`.ruby-gemset` file to specify the gemset. These files are supported
by RVM as documented [here](https://gist.github.com/fnichol/1912050).

.ruby-gemset

+mitx

.rvmrc

-rvm use 1.9.3-p374@mitx