Fall back to Ruby 2.1 parser if the current Ruby runtime is not supported in Parser

I've seen a CRuby developer tweeted about an inconvenience that RuboCop aborts running on Ruby head version. So I applied the almost same workaround [we've done before](https://github.com/bbatsov/rubocop/commit/657e91110c29c69b29f0db027289f3091651cfac) so that they can debug and improve the CRuby stability.