osx travis

[mattdowle]

To investigate #3939

• add to CRAN_Release.cmd procedures to turn back on OSX. I had turned it off in macOS / OSX builds in GLCI #3326 because it was slowing down dev cycle. Should be turned back on in release procedures.

[codecov]

Codecov Report

Merging #3941 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #3941   +/-   ##
======================================
  Coverage    99.4%   99.4%           
======================================
  Files          72      72           
  Lines       13643   13643           
======================================
  Hits        13562   13562           
  Misses         81      81

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