Pin an explicit version of django-filter (<2) on cookbook example

[NoumirPoutipou]

With requirements set with django-filter==0.11.0, the cookbook example is unable to execute query using filters. Leading to a TypeError :

__init__() got an unexpected keyword argument 'request'

Due to line 95 in filter/fields.py

Pin it to the same requirements as graphene-django itself.

[coveralls]

Coverage increased (+0.2%) to 94.667% when pulling a57098b on NoumirPoutipou:master into dc561c6 on graphql-python:master.