Updated ruby version to 2.0.0-p451

https://www.ruby-lang.org/en/news/2014/02/24/ruby-2-0-0-p451-is-released/
Necessary to compile native gem extensions for json gem on OSX
mavericks. Due to apple changing the compiler options. See
http://stackoverflow.com/questions/22352838/ruby-gem-install-json-fails-on-mavericks-and-xcode-5-1-unknown-argument-mul
and
http://stackoverflow.com/questions/22427515/set-ruby-version-in-gemfile