[CI] Reliability of s390x Travis CI build

[Wes McKinney]

I would be interested to know the empirical reliability of the s390x
Travis CI build, but my guess is that it is flaking at least 20% of
the time, maybe more than that. If that's the case, then I think it
should be added back to allow_failures and at best we can look at it
perioidically to make sure it's passing some of the time including
near releases. Thoughts?