I've asked bintray to add support for subkeys some weeks ago. No news since then, but they told me they were working on it. Problem right now is that you have to put your master gpg key on bintray to be able to sign: I wouldn't.
As soon as the subkeys are available on bintray, packages from the repo will be signed though :)
As soon as the subkeys are available on bintray, packages from the repo will be signed though :)