Bump djblets django dependency to 1.6
This is relatively straightforward. The changes here include:
* Bumping django to >= 1.6.1
* Replacing PIL with Pillow as the preferred image library. Existing PIL

  installations will continue to work.
* Remove obsolete call to setup_environ() in build-media.py
* Fix call to compile_messages() in build-i18n.py
* Changing some tests in the datagrid to account for the fact that 'timesince'

  now uses non-breaking spaces between the number and the unit.

Ran unit tests.
Installed djblets and saw that it used PIL if installed, and preferred Pillow

  if not.