Fallback for React 15 added

v4.0.0 was a complete rewrite that utilizes React v16 Portal API. However, we dropped support for older Reacts. That can be a problem for library authors that still want to support older Reacts.

So, version v4.1.0. adds a fallback in case that Portal API doesn't exist. Now you can use the same react-portal with React v16 and v15.