Exclude failing table tests from Travis (rebased onto develop)



This is the same as gh-2618 but rebased onto develop.

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As recently reported by @dominikl, @jburel and others, Travis intermittently fails with 2 of the table tests with an `Internal Exception`. This intermittent failure is captured by https://trac.openmicroscopy.org.uk/ome/ticket/12372. 

Until someone takes the time to investigate what goes wrong there, this PR marks these tests as `xfail` so that these intermittent failures do not affect our workflow as they do now.