Use MB_PYTHON_OSX_VER=10.9

Author: charris

.travis.yml

@@ -56,14 +56,17 @@ matrix:
       language: generic
       env:
         - MB_PYTHON_VERSION=3.5
+        - MB_PYTHON_OSX_VER=10.9
     - os: osx
       language: generic
       env:
         - MB_PYTHON_VERSION=3.6
+        - MB_PYTHON_OSX_VER=10.9
     - os: osx
       language: generic
       env:
         - MB_PYTHON_VERSION=3.7
+        - MB_PYTHON_OSX_VER=10.9
 
 before_install:
     - if [ "$TRAVIS_BRANCH" == "master" ]; then