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.