Replace .rvmrc by .ruby-version

Also relax the requirement. The .ruby-version represents
merely a recomendation for the Ruby version. The incompability
issues in the Ruby world are far less currently, than they were
when we introduced strict version enforcement. Thus strict
enforcement is no longer appropiate. Same holds for RubyGems.

This implicity also removes the recomendation on using RVM.
The .ruby-version file is supported by all major Ruby version
managers. Gemsets are no longer a needed feature since Bundler
handles multiple concurrrently installed Gems just well.

I also plan to drop or rework the RVM instructions in the installation guides, once I find the time for that.