Update mpeg4.json

It seems that Firefox 33 does not support MP4 videos.

See http://cl.ly/image/2I1D1h054745 (from https://twitter.com/sydlawrence/status/511887145678942208) and  [Mozilla Bug 941296](https://bugzilla.mozilla.org/show_bug.cgi?id=941296) which seems to indicate that mozilla34 is the target module for supporting MP4 (see also https://twitter.com/sydlawrence/status/511893154317230080).