Update osx_image to xcode9.2

.travis.yml

@@ -33,22 +33,27 @@ matrix:
       env:
         - MB_PYTHON_VERSION=3.6
     - os: osx
+      osx_image: xcode9.2
       language: generic
       env:
         - MB_PYTHON_VERSION=2.7
     - os: osx
+      osx_image: xcode9.2
       language: generic
       env:
         - MB_PYTHON_VERSION=3.3
     - os: osx
+      osx_image: xcode9.2
       language: generic
       env:
         - MB_PYTHON_VERSION=3.4
     - os: osx
+      osx_image: xcode9.2
       language: generic
       env:
         - MB_PYTHON_VERSION=3.5
     - os: osx
+      osx_image: xcode9.2
       language: generic
       env:
         - MB_PYTHON_VERSION=3.6