The `create-dev-env.sh` script now creates a gemset called edx-
platform, so make sure that this matches.

The right thing to do would be to have `create-dev-env.sh` peek at
this file and create a gemset with the appropriate name.

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).