Bump Ruby version in repo & on Heroku to latest 1.9.3

These changes specify the version of Ruby both for Heroku and for ruby version managers.

I'm updating conservatively since our next workshop is 3 weeks away.  But Ruby 1.9.2 has an [unpatched security vulnerability](http://www.ruby-lang.org/en/news/2013/05/14/taint-bypass-dl-fiddle-cve-2013-2065/).  Hence 1.9.3.

Referenced in issue https://github.com/railsbridge-boston/railsbridge-boston/issues/50.