Office 15 Build 15.0.2703.1000 has finally been leaked to the Internet. The information about the leak started on Twitter when @hb860 transferred the file to @AngelWZR who has posted on his website, that the file has been leaked.
The leak now appears to be out in the wild and represents an early build of the new Office platform. The leaked build is useable but MSwin.me states that it's somewhat unstable. This build is from the M2 group.
You can download the build 15.0.2703.1000 here.