Specify ruby gem versions

Summary:
With no gem version set, you get the latest, and the 2.x versions (up to
2.1.1) of uglifier cause khan-exercises/build/pack.rb to fail.

While the hood was up I also set versions on the other gems to avoid
future failures. I took the known good versions from the ci machine that
Mr Deploy is running on.

Test Plan: I ran the command manually as the ubuntu user on jenkins.khanacademy.org

Reviewers: desmond

Reviewed By: desmond

Differential Revision: http://phabricator.khanacademy.org/D2663