Run tests on multiple CPU cores

We can make the tests run slightly faster by parallelizing them across
multiple CPU cores:

- from some casual testing locally, 2 cores gives the optimum speedup
- Travis container-based builds have 2 CPU cores available[1]
- the net gain is about 20%, or 2 seconds
- unfortunately we can’t do this on the API because each test is still
  using the same instance of the database

1. https://docs.travis-ci.com/user/ci-environment/#Virtualization-environments

requirements_for_test.txt

@@ -3,6 +3,7 @@ pep8==1.7.0
 pytest==2.9.1
 pytest-mock==0.11.0
 pytest-cov==2.2.1
+pytest-xdist==1.14
 coveralls==1.1
 moto==0.4.23
 httpretty==0.8.14