Update Travis configuration to build 2.0.0.
Update Travis configuration to build 2.0.0.

The latest version of OS X ships with 2.0.0 Ruby. We should at least
build against it but allow for failures.

RallySoftware/boxen-git,rhoml/puppet-git,halyard/puppet-git,boxen/puppet-git,mavant/puppet-git,mavant/puppet-git,rhoml/puppet-git,RallySoftware/boxen-git,boxen/puppet-git