v4.0.0

The main breaking change in this release is that we no longer officially support Node v6 and v7. We've also updated @babel dependencies, which could also cause different behavior (plus fix a few bugs).

New feature:
If you don't have a package.json file in your project, you can drop an .importjs-root file in the root of the project to indicate to import-js where to resolve imports from. (#547 by @mikabytes)