Fix version regex in build_source.py

This was failing to handle hyphens in the upstream_version part.  From
the debian policy manual[1]:

> The upstream_version may contain only alphanumerics[36] and the
> characters . + - : ~ (full stop, plus, hyphen, colon, tilde) and
> should start with a digit. If there is no debian_revision then hyphens
> are not allowed; if there is no epoch then colons are not allowed.

[1] https://www.debian.org/doc/debian-policy/ch-controlfields.html#s-f-Version
Update godep package to build on trusty.

Trusty ships with a recent enough version of golang, so it's easiest to
build here.  The resulting binary package can then be used on both
trusty and precise because go binaries are very portable.

Note: the upstream godep project does not have any versioning at the
moment, so I've had to invent a version.  This has been generated by
locally tagging the first commit in the repository, and then using `git
describe --tags`
Merge pull request #60 from alphagov/godep_trusty

 Update godep package to build on trusty.