Migrated .rvmrc to the more modern .ruby-version and ruby-gemset

There is a new format that is supposed to work across all ruby version managing tools. Afaik it works for at least RVM and rbenv on up to date versions.

This PR deletes our .rvmrc and adds the equivalent .ruby-version and .ruby-gemset.

I also got word from @mpapis that it's way cooler ;-)

Would like feedback about whether or not this change is welcome.