To conserve our ESP test accounts' send quotas, don't run
the live API integration tests 13 times in every Travis run.
Instead, just run them twice, on a representative set
of Python/Django combinations:
* Once on Python 2.7 (currently with Django 1.8)
* Once on Python 3.x (currently 3.5 with Django 1.9)
(Prep for running weekly tests on Travis cron.)
The *non*-integration tests still run on all combos.
* Introduce RUN_LIVE_TESTS environment var to control
whether live API integration test cases should run.
Default True, except in Travis-CI runs default False.
* Enable RUN_LIVE_TESTS in .travis.yml matrix for the
Python/Django combos listed above.
Treat Mailgun metadata like all other ESPs: simple
key-value dict, where values are strings. If you want
to store JSON in metadata, you should serialize and
deserialize it yourself.
