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