check that configured cache sizes are >= 1

LRU cache implementation assumes that the cache is at least 1
element large. Catch invalid master configurations where cache
sizes less then 1 are specified.
Replace use of deprecated unittest.TestCase.assert_() with assertEqual/NotEqual etc methods

If newer version of unittest will be used, self.assertEqual(..., None)
should be replaced with self.assertNone(...), same with not equal to
None checks.
Merge branch 'replace_assert_in_CVSMaildirSource_tests' of git://github.com/vrutsky/buildbot
Merge branch 'check-cache-sizes' of git://github.com/elmirjagudin/buildbot