Test `omego upgrade` as part of the Travis build

This PR enables the `omego upgrade` command to be tested by the Travis build:
- download, set up and start a 4.4.8p1 server in the Travis pre-install step
- activate the `testUpgrade` integration test and run it as part of the test suite

Travis status should be 'success' meaning the test was run successfully. Upcoming steps can involve moving away the tests not requiring an OMERO instance into a separate Travis job and using Travis environments to test multiple upgrade conditions (Ice 3.3 -> Ice 3.5)

/cc @spli, @rleigh-dundee