Fixed the tox tests

- Added `PIP_INSECURE = 1` to all the Python 2.5 environments so packages would install.
- Changed the `py25` env so it uses the last version of Django that supports Python 2.5
- Changed the `py26` and `py27` envs so they use Django 1.5.1
- It should be noted that non of the environments that use Django 1.2.X pass anymore. This is most likely because of commit 6d5d9cc which added the widget `django.forms.widgets.ClearableFileInput` that was new in Django 1.3.X. Maybe easy-thumbnails should consider dropping support for Python 2.5 and Django 1.2.X.