The ElasticSearchSink currently compiles against es 0.19. Using 0.20 requires a recompile. I haven't tried 0.90 yet.
0.90 has been out for 2-ish weeks so we should change to compiling with it prior to 1.4 release.
I'm hoping to get priority for doing this in the next two weeks or so - however I have no issue with somebody else doing it.