svnpubsub: Set the media type to a unique one now that we finally agreed.

[in tools/server-side/svnpubsub]
* svnpubsub/server.py
  (Client.write_start): Use application/vnd.apache.vc-notify+json



git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1490045 13f79535-47bb-0310-9956-ffa450edef68