travis: update mac machine

Author: efiop

.travis.yml

@@ -48,9 +48,9 @@ jobs:
   - name: "3.7 on Linux"
     language: python
     python: 3.7
-  - name: "3.6 on Mac"
+  - name: "3.7 on Mac"
     os: osx
-    osx_image: xcode9.4
+    osx_image: xcode11.3
   # build jobs
   - name: "Mac pkg"
     stage: build