loom/puppet.py - Ubuntu 14.04 'rubygems' becomes 'ruby', handle error

Starting with at most Ubuntu 14.04 rubygems has become ruby. Attempting
to install rubygems results in an error that suggests re-installing
as ruby. I'm pretty sure this must have changed in a previous Ubuntu
version, since 14.04 is just an LTS cut of some previous version.

Rather than detect return codes I just use a context manager to
aggressively attempt to install rubygems and ruby. This results in
some cruft to stdout but is easier to understand than conditional
processing based on return codes.
Merge pull request #34 from asimihsan/ubuntu-14.04-rubygems

loom/puppet.py - Ubuntu 14.04 'rubygems' becomes 'ruby', handle error