pin the ruby version to the latest 1.9.3

without pinning the ruby version, running `bundle exec rake spec`
uses the system ruby to run rspec, and because of [an issue in `rbenv`](https://github.com/sstephenson/rbenv/pull/372)
this doesn't work properly in the dev VM. The `rbenv` bugfix hasn't
been released as of this time.
Merge pull request #29 from alphagov/pin-ruby-version

pin the ruby version to the latest 1.9.3 point release