Html.empty replaced by HtmlFormat.empty

See: https://groups.google.com/forum/#!msg/play-framework/bTvJbeR_zvU/mpom08-idCQJ and https://github.com/playframework/twirl/blob/master/api/src/main/scala/play/twirl/api/Formats.scala#L77
Merge pull request #3566 from jmcejuela/patch-2

Html.empty replaced by HtmlFormat.empty