ARROW-6987: [CI] Travis OSX failing to install sdk headers

[mrkn]

macOS_SDK_headers_for_macOS_10.14.pkg is no longer provided in Xcode 11.
We shouldn't depend on it.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-6987

[kou]

macOS job is green: https://travis-ci.org/apache/arrow/jobs/604734693

We can merge this once all CI is green.

[codecov-io]

Codecov Report

Merging #5756 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #5756 +/- ##
==========================================
- Coverage 88.97% 88.96% -0.02% 
==========================================
Files 995 995 Lines 135328 135328 Branches 1501 1501 ==========================================
- Hits 120408 120394 -14 - Misses 14555 14569 +14 
Partials 365 365

Impacted Files	Coverage Δ
cpp/src/arrow/filesystem/s3_internal.h	90.74% <0%> (-3.71%)	⬇️
cpp/src/arrow/json/converter.cc	90.05% <0%> (-1.76%)	⬇️
cpp/src/arrow/json/chunked_builder.cc	80% <0%> (-1.67%)	⬇️
cpp/src/arrow/csv/column_builder.cc	95.54% <0%> (-1.49%)	⬇️
cpp/src/arrow/util/task_group_test.cc	98.86% <0%> (-1.14%)	⬇️
python/pyarrow/tests/test_plasma.py	95.76% <0%> (-0.32%)	⬇️
python/pyarrow/tests/test_parquet.py	95.3% <0%> (+0.05%)	⬆️
python/pyarrow/plasma.py	56.16% <0%> (+1.36%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 37434fb...6c7d156. Read the comment docs.