Fix errors when running migrations against MySQL 5.1.72

Fixes the following type of error:
Error in migration: askbot:0055_auto__chg_field_question_thread
ValueError: The database backend does not accept 0 as a value for AutoField.
Allow Django 1.5.x, not just 1.5 itself

setup.py says that "Django 1.3.1 - 1.5.1" are supported but REQUIREMENTS in __init__.py only allows up to 1.5.
Merge branch 'migrate_mysql_fix' into bright
Merge branch 'allow_django_15x' into bright