Django 2.1.* support #46

Hi!

Currently, we faced the following issue for Django 2.1.* series:

django_babel/extract.py", line 2, in <module>
    from django.template.base import Lexer, TOKEN_TEXT, TOKEN_VAR, TOKEN_BLOCK
ImportError: cannot import name 'TOKEN_TEXT'

Package versions:

• Django==2.1.5
• Babel==2.6.0
• django-babel==0.6.2

So there need to add some compatibility. I'm in progress with the PR now.