use xenial dist in travis ci

[andyxning]

What this PR does / why we need it:
This PR updates travis ci distribution to 16.04.
Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

[andyxning]

FYI: https://blog.travis-ci.com/2018-11-08-xenial-release

[brancz]

neat 👍

/lgtm

as a side note we might want to start looking into kind, I've seen great speed-ups in other setups, but also if this works for us I don't see a huge reason to change right now

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[andyxning]

as a side note we might want to start looking into kind, I've seen great speed-ups in other setups, but also if this works for us I don't see a huge reason to change right now

It seems that kind is not so general as minikube. Since minikube v1.0.0 has been released and it should be stable enough, pinning on minikube should be ok for now.