dev-java/icedtea-web-1.5 was released on 2014-04-07.
dev-java/icedtea-web-1.5.1 was released on 2014-08-15.
Just renaming the current icedtea-web-1.4.2-r1.ebuild and removing the patch for parallel install (not necessary anymore) should work (it did for me).
Created attachment 387634 [details] icedtea-web-1.5.1.ebuild
Added, thanks.