Mandrill backend blows up hard if settings.INBOUND_MANDRILL_AUTHENTICATION_KEY is missing

[stevejalim]

TBC whether that's a deliberate design decision or not (the example config implies it should be added, but the readme for the project makes it sound a bit optional.

A fix would simply be to change https://github.com/yunojuno/django-inbound-email/blob/master/inbound_email/backends/mandrill.py#L122

from

        if settings.INBOUND_MANDRILL_AUTHENTICATION_KEY:

to

        if hasattr(settings, 'INBOUND_MANDRILL_AUTHENTICATION_KEY'):