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.

Update .travis.yaml to have updated bintray api keys
Update bintray-release.json to have correct casing for repo

projects.

Update the appveyor configuration to no longer use mujx keys.

Travis-CI keys still need to be updated.