fix(element): Fix typing of ElementFinder.then

Author: massimocode

Diff for: lib/element.ts

@@ -792,7 +792,9 @@ export class ElementArrayFinder extends WebdriverWebElement {
 export class ElementFinder extends WebdriverWebElement {
   parentElementArrayFinder: ElementArrayFinder;
   elementArrayFinder_: ElementArrayFinder;
-  then: (fn: Function, errorFn?: Function) => wdpromise.Promise<any> = null;
+  then?:
+      (fn: (value: any) => {} | wdpromise.IThenable<any>,
+       errorFn?: (error: any) => any) => wdpromise.Promise<any> = null;
 
   constructor(public browser_: ProtractorBrowser, elementArrayFinder: ElementArrayFinder) {
     super();