Switch back to Jenkins for MacOS builds

[njsmith]

While the Travis MacOS infrastructure is clearly much better than it
used to be, doing these tests on Jenkins is still faster overall, and
it avoids hitting gh-584.

This commit:

• adds a temporary workaround for
  Apparent segfault in immutables.Map.__init__ MagicStack/immutables#7
• re-enables all MacOS builds on Jenkins
  • including 3.7, which was previously not enabled
• re-disables Travis MacOS builds

[codecov]

Codecov Report

Merging #585 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #585   +/-   ##
=======================================
  Coverage   99.27%   99.27%           
=======================================
  Files          89       89           
  Lines       10640    10640           
  Branches      747      747           
=======================================
  Hits        10563    10563           
  Misses         59       59           
  Partials       18       18

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[pquentin]

Thanks for your thankless work on CI!