Fix TypeError from trying to call a boolean

[Diftraku]

user.is_authenticated is a boolean value in Django 2.0, this causes a TypeError when trying to load the page with adminlte2 templates.

Example end of stack trace for loading the admin site:

File "/usr/lib/python3.6/site-packages/django_adminlte/templatetags/adminlte_helpers.py" in avatar_url
  20.         hash=md5(user.email.encode('utf-8')).hexdigest() if user.is_authenticated() else '',

Exception Type: TypeError at /admin/
Exception Value: 'bool' object is not callable

[adamcharnock]

Thanks for the PR @Diftraku, it is much appreciated. In the end I added a compat.py file with is_authenticated() which detects the appropriate course of action.

[adamcharnock]

I've just released 0.2.2 containing this fix