Disable s390x and arm64 tests on travis for now.

[trivialfis]

(#6638) https://travis-ci.org/github/dmlc/xgboost/builds/756313309
(#6640) https://travis-ci.org/github/dmlc/xgboost/builds/756317012

[hcho3]

I hope we can re-enable the arm64 test before the next release.

[hcho3]

If not, we'll have to fire up an ARM64 worker on AWS and manually build the binary wheel.

[trivialfis]

If not, we'll have to fire up an ARM64 worker on AWS and manually build the binary wheel.

That would be quite a headache. I don't know how the monitor it, perhaps we can just merge PRs ignoring the related failures instead of disabling the tests completely?

[trivialfis]

@hcho3 WDYT?

[hcho3]

We can re-enable it once the 1.4 release gets closer

[hcho3]

Adding ARM64 on Jenkins should be quite easy actually. It's just going to burn our CI budget faster.

[trivialfis]

@hcho3 Is Windows still a bottleneck in CI spending?

[hcho3]

Not any more. Now we need more of across-the-board spending cut.

[hcho3]

Now the majority of the cost is attributed to Linux.

[hcho3]

The use of multi-GPU instances incurs the highest amount of cost

[hcho3]

So adding ARM64 to the matrix won't be too bad. Most of the cost comes from GPU-enabled instances.

[hcho3]

Let me file a small PR today to add ARM64 worker to Jenkins.