Set last_login on create_user for backward compatibility

`User.last_login` default is removed from django > 1.7.0. For existing
project using `django-user-management` project migrations would be run
after `django.contrib.auth` migrations and will cancel last_login `IS
NULL`.
Update CHANGELOG.md [ci skip]
Update assert for last_login
Merge pull request #66 from incuna/fix-last-login-not-null

User.last_login should be set on _create_user