Drop `from __future__ import unicode_literals`;
it was there for Python 3.2 compatibility (which
Anymail doesn't support). Ensures tests use normal
strs in Python 2.x.
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.
