requirements: use twl djmail fork

forked djmail will provide features required for mass emailing via WMF
Cloud VPS.

Bug: T412427

Author: jsnshrmn

requirements/base.txt

@@ -16,7 +16,7 @@ django-filter==24.3
 django-premailer==0.2.0
 django-reversion==5.1.0
 djangorestframework==3.15.2
-djmail==2.0.0
+git+https://github.com/WikipediaLibrary/djmail/tree/Jsn.sherman/T412427
 factory_boy>=2.10.0,<4.0
 jsonschema==3.2.0
 mwoauth==0.4.0