Add support for Electron 2.x

[jarek-foksa]

The full list of breaking Electron 2.x API changes is on https://electronjs.org/docs/tutorial/planned-breaking-changes

For example, nightmare.screenshot() is currently broken because it calls img.toPng() instead of img.toPNG().

[wiliam-paradox]

@jarek-foksa I made a pull request for the .toPNG #1454