packaging: require mercurial >= 4.5
Otherwise building the doc package fails with:
(third party extension evolve requires version 4.5 or newer of Mercurial; disabling)
Exception occurred:
File "/usr/lib/python2.7/dist-packages/mercurial/help.py", line 624, in help_
raise error.Abort(msg, hint=hint)
Abort: no such help topic: evolve