Cabot includes an option for SSL, but requirement missing from cabot-alert-email

[dbuxton]

See conversation at the end of arachnys/cabot#4

Basically our default settings are for SES which requires (as I recall, maybe it's changed) SSL (and thus https://github.com/bancek/django-smtp-ssl) for SMTP.

However our default Cabot requirements.txt doesn't include this package and neither does this repo.

Solution: add dependency for django-smtp-ssl to setup.py of this plugin.

(This problem also exists with e.g. https://github.com/cabotapp/cabot-alert-twilio/blob/master/setup.py although in that case we include in the cabot requirements: https://github.com/arachnys/cabot/blob/af66cc806d516239281e35226f43a2dc4dbef18c/requirements.txt#L43)

[frankh]

Django now supports SSL in django.core.mail.backends.smtp.EmailBackend, and django-smtp-ssl hasn't been updated in years, so I don't think switching back to that is the solution