Update pytest from 2.3.5 to 2.4.2

This gets us yield_fixture:

    http://pytest.org/latest/yieldfixture.html
Change order of test deps installs

The pytest library depends on the py library, but the latter was being
installed after the former (therefore it was being installed twice).
Implement mk as a pytest fixture

And convert test_website to use it.
Fix two more test files to use mk from fixture
Switch to mk fixture for the rest of the tests
Merge pull request #252 from gittip/fixture

implement a pytest fixture around fsfix.mk