Build gems for ruby-2.0.0

This PR adds ruby-2.0.0 to the `ruby::all::gem` define, such that it
will compile and install the gem for ruby 2.0.0

I've tested this with the `zeus` gem, but it might not work for all
gems. I'd appreciate some testing of by anyone who uses the following
classes:

- gds-development
- git-pulls
- git-remote-branch
- github_gem
- travis
- vagrant-dns
- vagrant-vbguest
- vagrant_gem
- zeus
Merge pull request #45 from alphagov/rubygems-for-ruby-200

Build gems for ruby-2.0.0