Setup toolchain to build on a trusty VM

Also update docs to refer to Precise and Trusty as opposed to Lucid and
Precise.

This will make the packager VM run trusty instead of precise, so that
the more recent tools will be available to build_source.py.  The actual
builds still happen with a schroot for the appropriate distro (or at
launchpad...).
Merge pull request #56 from alphagov/build_on_trusty

Setup toolchain to build on a trusty VM