Change Ruby gemset to edx-platform

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.
Merge pull request #31 from edx/nate/edx-platform-ruby-gemset

Change Ruby gemset to edx-platform