Update OS X travis images to xcode7.3

This will be the new default OS X image on Travis soon

Author: iMichka

.travis.yml

@@ -90,13 +90,13 @@ matrix:
             # gcc 5.3.0
             - g++-5
     - os: osx
-      osx_image: xcode7.2
+      osx_image: xcode7.3
       language: generic
       env:
         - XML_GENERATOR="castxml"
         - TRAVIS_PYTHON_VERSION="2"
     - os: osx
-      osx_image: xcode7.2
+      osx_image: xcode7.3
       language: generic
       env:
         - XML_GENERATOR="castxml"