Travis should install SendGrid deps

[ci skip]  - they're the same as Mailgun deps

.travis.yml

@@ -35,6 +35,6 @@ install:
   - pip install $DJANGO
   # For now, install all ESPs and test at once
   # (in future, might want to matrix ESPs to test cross-dependencies)
-  - pip install .[mailgun,mandrill]
+  - pip install .[mailgun,mandrill,sendgrid]
   - pip list
 script: python -Wall setup.py test