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`

This PR also includes a fix for `build_source.py` to support hyphens in an upstream version number.