Revert "CI: OSX - Use Xcode7.3"

This reverts commit ca835ddfd2c021bc72985d06b62a58c5437ff57b.

.travis.yml

@@ -20,7 +20,6 @@ env:
 matrix:
   include:
     - os: osx
-      osx_image: xcode7.3
       env:
        - CMAKE_PREFIX_PATH=/usr/local/opt/qt5/lib/cmake
        - CEF_BUILD_VERSION=3.3112.1656.g9ec3e42