Depend on unittest2.

unittest2 has backports of discover etc which allows us to make it a
stock feature rather than something that is present sometimes, and
which we have to carry ugly monkey patches for.

Change-Id: Ib30a4a9de8c58edf5d9c4ea5e06d49b0c821f12f
Fixup travis. OOPS.

Change-Id: Ia03284a3ca79c66e0f1e56a526c6b18a70aca687
Improve support for discovery on testtools.tests.

Our tests currently import test_with_with into test_testcase so that
we could still run tests on python2.5. Since we no longer support
2.5, we don't need this hack. Its a hack because discovery will
import test_with_with directly itself, and thus we triggered
a duplicate test id error when attempting to use discover on the
testtools test base.

Change-Id: I027929ab4b0df350dd18ab01ffd5c493fe35e101