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.