Koop 4.0.0 released

We have released Koop 4.0.0. There are no new features, but we have dropped support for Node.js < 6.4.0. By only supporting more recent versions of Node, we can leverage some of the great features of modern javascript. We can also skip the Babel translation step, so we no longer need to ship Koop with a dist directory. Koop 4.0.0 also starts out with a much refactored provider registration routine. Usage has not changed, but provider status and debugging are much improved.