Re-enable macOS testing on Travis

Since Jenkins is acting screwy and not letting people see the logs
:-(.

It's possible that this will cause python-triogh-584 to start happening again,
so I guess we'll just have to watch out for that...

Fixes python-triogh-749

.travis.yml

@@ -28,15 +28,15 @@ matrix:
     - python: 3.8-dev
       dist: xenial
       sudo: required
-    # - os: osx
-    #   language: generic
-    #   env: MACPYTHON=3.5.4
-    # - os: osx
-    #   language: generic
-    #   env: MACPYTHON=3.6.6
-    # - os: osx
-    #   language: generic
-    #   env: MACPYTHON=3.7.0
+    - os: osx
+      language: generic
+      env: MACPYTHON=3.5.6
+    - os: osx
+      language: generic
+      env: MACPYTHON=3.6.7
+    - os: osx
+      language: generic
+      env: MACPYTHON=3.7.1
 
 script:
   - ci/travis.sh