make it possible to install and use older ruby 2.1 versions

We have a ton of apps which still use ruby 2.1.0 (with railsexpress patches applied) and which can't be migrated over night.

Problem is we can't use recent rvm versions anymore because rvm complains about unknown ruby version 2.1.0.

This patch adds 2.1.0 and 2.1.1 back to the list of known ruby versions.

This fixes #2990