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.

Testing done:
* Ran unit tests.
* Installed djblets and saw that it used PIL if installed, and preferred Pillow
  if not.

Reviewed at https://reviews.reviewboard.org/r/5309/