Use dpkg for all should_build.py version comparisons

Prior to this commit, we were using dpkg to compare the archive
version against the manifest version. However, we weren't using it to
compare versions from different archive pockets, which meant that once
precise-updates' kernel ticked up to 3.2.0.100.something, it started
comparing lower than the release's 3.2.0.23.something, meaning automated
promotion stopped happening.

This should address that issue.