Use .ruby-version and allow for overrides

This should fix issue #693. Motivation for this patch is included in that issue and repeated here for convenience:

1. The installation instructions are currently recommending a specific version of Ruby. This can be enforced using a .ruby-version file.
2. A developer might want to use a .ruby-gemset file as well, but this needs to be ignored by git unless all developers agree on what the gemset name should be.
3. These settings can be overridden by a developer using a specific .rvmrc file, which has to be ignored by git as well.

I signed the individual CLA as "Jacopo Notarstefano" using GitHub username "jacquerie".