requirements: Update django debug toolbar to 1.5

requirements: Update django debug toolbar to 1.5

This is the latest version. This breaks compatibility with Django 1.7,
but this version is unsupported and should not be used for development.
We do, however, continue to test support for Django 1.7 and 1.6.

This also prevents the need to manually track sqlparse dependency [1].

[1] https://github.com/jazzband/django-debug-toolbar/issues/856

Signed-off-by: Stephen Finucane <stephenfinucane@hotmail.com>