Fix problem that "previous" references don't reference previous snapshot versions.
Fix problem that "previous" references don't reference previous snapshot versions.

The problem is that we're not actually advancing the timestamp back in the
loop; we're just repeating the same calculatin N times.

Note this is a WIP - since my machine is currently kinda broken, I cannot run
the tests myself, thus I'm abusing the PR to let Travis do the job.

ganwell/pyaptly,adfinis-sygroup/pyaptly