ARROW-5660: [GLib][CI] Use Xcode 10.2

[kou]

No description provided.

[codecov-io]

Codecov Report

Merging #4641 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4641   +/-   ##
=======================================
  Coverage   88.49%   88.49%           
=======================================
  Files         890      890           
  Lines      110406   110406           
  Branches     1418     1418           
=======================================
  Hits        97703    97703           
  Misses      12422    12422           
  Partials      281      281

Impacted Files	Coverage Δ
cpp/src/arrow/io/readahead.cc	95.91% <0%> (-1.03%)	⬇️
cpp/src/plasma/thirdparty/ae/ae.c	70.75% <0%> (-0.95%)	⬇️
cpp/src/arrow/csv/column-builder.cc	97.05% <0%> (+1.76%)	⬆️

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[wesm]

+1