Install Mailgun's (and potentially, other ESPs') dependencies with Anymail
Rationale: this is a [generic way](https://github.com/anymail/django-anymail#anymail-1-2-3) of handling Anymail ESP dependencies
@@ -14,4 +14,4 @@ raven==6.8.0 # https://github.com/getsentry/raven-python
# Django
# ------------------------------------------------------------------------------
django-storages[boto3]==1.6.6 # https://github.com/jschneier/django-storages
-django-anymail==2.2 # https://github.com/anymail/django-anymail
+django-anymail[mailgun]==2.2 # https://github.com/anymail/django-anymail
