Make it easy to try production builds from Travis

[gmarty]

Travis makes production builds on each push but we have no way to get these builds at the moment.

That would be nice if we could upload each build to a server so we can easily test them. Then maybe we can remove builds older than say a week (or less depending on the disk space).

Maybe we can also reuse this mechanism to upload the Android and iOS builds from the latest master branch somewhere and call it nightly.

As discussed with @samgiles.

[wilsonpage]

FYI we do already build nightlies. You can find them in Github Releases. But correct, we don't have any way of getting builds per push.