Tests: add MIDDLEWARE_CLASSES for Django 1.7
Unpin elasticsearch library version for testing
Tests: Whoosh test overhaul

* Move repetitive filesystem reset logic into WhooshTestCase which
  cleans up after itself
* Use mkdtemp instead of littering up the current directory with a
  'tmp' subdirectory
* Use skipIf rather than expectFailure on test_writable to disable
  it only when STORAGE=ram rather than always