tox.ini updates for Python 2.5, Django 1.5

* remove Python 2.5 support

  Since it's already impossible to run these tests (see
  https://bitbucket.org/hpk42/tox/issue/91/python-25-support) there's no
  point in keeping increasingly obsolete, always-failing tests around.

* add targets for Django 1.5
* avoid hard-coding minor Django versions