Change MappingType.get_indexes to get_index

From the point of view of a mapping type, it should always go in one
index. This codifies that and changes the method name so it's clearer
and matches DjangoMappingType.
Move django tests to elasticutils/contrib/django/tests/

That's a better place for them. Then we can split them up into
smaller files.
Clean up django tests; make django requirement for dev

This nixes all the code that allowed you to run tests without having
django installed. Now, in order to run the tests, you need to have
it installed--it's a requirement for developing elasticutils but
not using elasticutils.

This also breaks up test_django.py into a bunch of separate files
and moves the infrastructure to __init__.py.
Rework django tests to use ElasticSearchTestCase

* reworks all the django tests to use ElasticSearchTestCase
* fixes bugs in ElasticSearchTestCase
* adds some additional methods that were helpful to have around
Tweak requirements files for better Travis