Add Ubuntu Trusty Tahr support to install script.

Regarding the removal of rubygems - of the things used for this install script, mapit installs system packaged sass, alaveteli and theyworkforyou install rubygems separately, pombola can switch to using bundler after https://github.com/mysociety/pombola/pull/1558 (I assume as there's only one packages file it's only debian wheezy), and fixmystreet has more generic Ubuntu support and more in https://github.com/mysociety/fixmystreet/pull/923. I don't think anything else is needed for this script to support trusty, though currently only FixMyStreet with https://github.com/mysociety/fixmystreet/pull/923 will support it.