Try another XCode version for Travis CI OSX build

.travis.yml

@@ -2414,7 +2414,7 @@ env:
     # Make a separate matrix build that only makes the OS X package
     - MACOSX_PACKAGE=1
 
-osx_image: xcode7.2
+osx_image: xcode6.4
 
 addons:
   apt: