Use Python 3.6.1 on MacOS

It seems that 3.6.0 has a bug that causes
https://github.com/nedbat/coveragepy/issues/703 ("Couldn't use data file
'/Users/travis/build/zopefoundation/persistent/.coverage': Safety level
may not be changed inside a transaction" and a failed build from
'coverage run').

.travis.yml

@@ -70,7 +70,8 @@ jobs:
     - name: Python 3.6 wheels for MacOS
       os: osx
       language: generic
-      env: TERRYFY_PYTHON='macpython 3.6.0'
+      # NB: 3.6.0 causes https://github.com/nedbat/coveragepy/issues/703
+      env: TERRYFY_PYTHON='macpython 3.6.1'
     - name: Python 3.7 wheels for MacOS
       os: osx
       language: generic