contint: python3.4 on Trusty labs slaves

pywikibot has a test suite for python3.4, so we better have it installed
on the Ubuntu Trusty Jenkins slaves.  (3.4 is not available on Precise).

Install python3.4-dev to be able to compile C python modules.

Change-Id: Ic01a876b96e78f5c94938a2b46f1737ae8aee2a5