diamond: install python-statsd on >= precise

backporting to lucid is a very small gain for a lot of pain (e.g. no debhelper
9, no python 2.7 by default) and we have very few lucid hosts left, namely:

sodium.wikimedia.org
ms1004.eqiad.wmnet
nescio.esams.wikimedia.org

Change-Id: Ia8b7773c818a6396517497fd050f69a5ea675b8c