Specify a later version of rubygems

Version 1.8.15 from backports should not have the Syck bug described
here:
http://stackoverflow.com/questions/9004804/invalid-gemspec-illformed-requirement-yamlsyckdefaultkey0xb5f9c990-3

http://blog.rubygems.org/2011/08/31/shaving-the-yaml-yak.html
Assume Ubuntu 12.04 (precise) not 10.04 (lucid)

Precise is now the long term maintenance version, and has a more
up-to-date package of rubygems available.
Add note about possible errors on rubygems upgrade.