Add Django 1.5 and 1.6 to tox.ini

Both Django 1.5 and 1.6 are supported and should be tested against.
This patch adds the two corresponding configurations to tox.ini.

Change-Id: I72db0a1643da17bc5c35f863cb5e3b4b81fffab8
Closes-bug: #1306074
Merge "Add Django 1.5 and 1.6 to tox.ini"