Django 2.0 support

[smajda]

A simple change: adding the now-required on_delete to the ForeignKeys.

One test fails though:

FAIL: test_send_attachments (backend_tests.DBEmailBackendTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/jon/.virtualenvs/bravo20/src/django-db-email-backend/db_email_backend/tests/backend_tests.py", line 56, in test_send_attachments
    self.assertEqual(attachment.mimetype, 'application/octet-stream')
AssertionError: 'text/markdown' != 'application/octet-stream'
- text/markdown
+ application/octet-stream

I'm not sure if the test is wrong (it's a markdown file, seems like 'text/markdown' would be correct...) or if it is supposed to be 'application/octet-stream' for some reason instead.

[jsatt]

Thanks @smajda. Merged and released 0.4.

[smajda]

Thanks for the quick merge and release!