List python_cpp as failing on OS X

.travis.yml

@@ -67,5 +67,9 @@ matrix:
       env: CONFIG=ruby22
     - os: osx
       env: CONFIG=jruby
+    # https://github.com/google/protobuf/issues/1253 - Started failing when
+    # we moved to an OS X image that is 10.11.
+    - os: osx
+      env: CONFIG=python_cpp
 notifications:
   email: false