Change version matcher in extconf to be regex for 2.1 releases

see https://www.ruby-lang.org/en/news/2014/02/24/ruby-2-1-1-is-released/
and https://www.ruby-lang.org/en/news/2013/12/21/semantic-versioning-after-2-1-0/
and https://github.com/cldwalker/debugger-ruby_core_source/pull/32

This is probably just a stop gap and something longer term is needed