ArrayField converter broke in Django 1.8.*

[fabiocerqueira]

Guys,

JSONField was added just in Django 1.9[1] and in graphene code try import that as version 1.8.*
This could cause problem to convert_postgres_array_to_list, because if I'm using django 1.8 graphene_django will try import JSONField causing a ImportError and broke all the others imports.

A possible solution below:

try:
    # Postgres fields are only available in Django 1.8+
    from django.contrib.postgres.fields import ArrayField, HStoreField, RangeField
except ImportError:
    ArrayField, HStoreField, RangeField = (MissingType, ) * 3


try:
    # Postgres fields are only available in Django 1.9+
    from django.contrib.postgres.fields import JSONField
except ImportError:
    JSONField = MissingType

[1] - https://docs.djangoproject.com/en/1.9/releases/1.9/
[2] - https://github.com/graphql-python/graphene-django/blob/master/graphene_django/compat.py#L23