Add support for protractor 4.x

[floribon]

Some breaking changes have been introduced since protractor 4.0.0 that this PR fixes.

It also allows the user to return this.quit() since cucumberjs accepts promises instead of forcing the use of a callback.

Contributions to this fork are welcome until this has been merged here.

[floribon]

Note: if you are using protractor 5.x instead of 4x, replace these lines:

// Syntax for protractor 4.x
var browser = protractor.ProtractorBrowser.wrapDriver(driver);

// Syntax for protractor 5.x
var browser = new protractor.ProtractorBrowser(driver);