travis: Test only Mac OS 10.11

Author: 1st1

.travis.yml

@@ -11,7 +11,7 @@ env:
 matrix:
     include:
         - os: osx
-          osx_image: xcode7.1  # OS X 10.10
+          osx_image: xcode7.2  # OS X 10.11
           # Travis macOS env does not support Python yet,
           # so we have to set things up manually in install.sh.
           env: PYTHON_VERSION=3.5.2