Run installed django-admin.py

We need to run the installed django-admin.py, not django-admin installed
by distribution in /usr/bin.

Fixes #625

Signed-off-by: Michal Čihař <michal@cihar.com>
Install latest Django 1.6 for migration

We don't want to stick with some old version here, let's use latest from
1.6 branch.

Signed-off-by: Michal Čihař <michal@cihar.com>