Use Python 3.6.2 on Mac OS

3.6.1 broke ABI, 3.6.2 fixed it.  More details at
https://bugs.python.org/issue29943.

.travis.yml

@@ -71,7 +71,9 @@ jobs:
       os: osx
       language: generic
       # NB: 3.6.0 causes https://github.com/nedbat/coveragepy/issues/703
-      env: TERRYFY_PYTHON='macpython 3.6.1'
+      # NB: 3.6.1 had that ABI regression (fixed in 3.6.2) and would be a bad
+      # version to use
+      env: TERRYFY_PYTHON='macpython 3.6.2'
     - name: Python 3.7 wheels for MacOS
       os: osx
       language: generic