add missing Promise to IRoot
add missing Promise to IRoot

dsebastien/DefinitelyTyped,dsebastien/DefinitelyTyped,borisyankov/DefinitelyTyped,markogresak/DefinitelyTyped,georgemarshall/DefinitelyTyped,mcliment/DefinitelyTyped,georgemarshall/DefinitelyTyped,borisyankov/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped