* Upload nightlies to a matrix room

* Add branch to upload name

* Update README with new nightly location

Try to prevent node upgrade via homebrew

I think the most recent build broke deployments?

Add new key for ubuntu repo that causes travis to fail.

Apparently the nlohmann library is not available in the standard
repositories.  I would rather build it than add universe deps.

Update JSON build dependencies, change the dockerfile and other
CI files that relate to this.  Also update target mtxclient
revision.