Specify ruby version in Gemfile instead of a named rvm installation

A small tweak for the ruby version - I'm using chruby and happen to have named my ruby 2.0.0 installation 'ruby200' instead of '2.0.0'.  My goal here is to move the soft requirement of a matching ruby installation name into an explicit ruby version requirement in the Gemfile.

See http://bundler.io/v1.2/gemfile_ruby.html for the ruby declaration, bundle check will report 
````
Your Ruby version is 1.9.3, but your Gemfile specified 2.0.0
````
if the current ruby version does not match.