Prefix deb version with 0.

Otherwise we're going to end up with some very large integers. Prefixing
with `0.` will allow us to switch to a different scheme, or stabilize, in
the future.

We could also append `${GIT_COMMIT}` to this, should we wish to track that.
Merge pull request #4 from alphagov/prefix_deb_version

Prefix deb version with 0.