.travis.yml (take 2) and test fixes (rebased onto dev_4_4)



This is the same as gh-941 but rebased onto dev_4_4.

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Changes for activating Python tests within .travis.yml.  Primarily tests have been removed or fixed, though the use of OMERODIR in `src/omero/cli.py` was required for some tests. All tests pass locally, though some are still failing on travis. Opening PR to prevent conflicts in the test classes in other PRs.