Fix failing Travis CI on Mac

[hcho3]

Mac builds on Travis CI are timing out, e.g. https://travis-ci.org/dmlc/xgboost/jobs/485689143#L2365

Solution:

1. Use the latest Mac image with XCode 10.1, so that we don't have to upgrade XCode and Command Line tools manually
2. Use Homebrew Addon integration of Travis CI, instead of manually invoking brew install. Should result in faster CI runtime.

[hcho3]

@CodingCat

[hcho3]

@CodingCat Is it fair to assume that XGBoost4J doesn't yet work with Java 10?

[CodingCat]

yes, it doesn't work with anything beyond JDK8 (at least didn't test)

[CodingCat]

and Spark doesn't work with JDK8+, so.....once Spark moves, we can move accordingly

[hcho3]

Ah, I see, we'll have to stick with JDK 8 then.

[hcho3]

@trivialfis Lint is failing after I modified the build enviornment: https://travis-ci.org/dmlc/xgboost/jobs/486002965#L2420. Is clang-tidy suddenly finding naming convention violations that it didn't find before?

[hcho3]

@CodingCat Hmm, interesting, there is a talk about Spark 3.0 release: http://apache-spark-developers-list.1001551.n3.nabble.com/time-for-Apache-Spark-3-0-td23755.html. Are you aware of major motion towards this?

[CodingCat]

yes, next major release is Spark 3.0

the idea in my mind is that, before upgrading Spark, I need to come up several tests for model compatibility, and behavior consistency....

[hcho3]

@trivialfis I'm looking at #4034 to fix the lint issue. Hopefully we can get unblocked again.

[hcho3]

I've removed clang-tidy for the time being. We'll add it back in #4034.

[codecov-io]

Codecov Report

Merging #4086 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4086   +/-   ##
=======================================
  Coverage   60.56%   60.56%           
=======================================
  Files         130      130           
  Lines       11756    11756           
=======================================
  Hits         7120     7120           
  Misses       4636     4636

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