-e: plugins of versions earlier than 5.3 can't play 5.3 content but 5.3 plugins can play all content

-repro:
--build an empty scene with 5.3
--install a webplayer plugin from http://unity3d.com/webplayer (currently 5.2.3)
--try to run you content
--NOTICE it says "Invalid data file', details: 'Data file is corrupt (not a Unity Web Player file)"
--install a 5.3 plugin (not the closed network one, Beta channel has 5.3.0f3 runtime)
--run your content
--NOTICE it now plays fine without an error

-workaround: install the latest 5.3 plugin, we can increase the min version to 5.3 on release