Update .ruby-version

Ruby has fixed some security issues in v2.0.0 since -p0:
* http://www.ruby-lang.org/en/news/2013/05/14/ruby-2-0-0-p195-is-released/
* http://www.ruby-lang.org/en/news/2013/06/27/hostname-check-bypassing-vulnerability-in-openssl-client-cve-2013-4073/

I would recommend not using a patchlevel in .ruby-version, to avoid sticking with an unsecure version of ruby.