Add event listener functions for compatibility with rxjs 5

.fromEvent() is throwing an error because the mock isn't compatible with the event listener type. If we keep running into changes in this, we might want to consider changing the implementation. Unless and until we get to that point, this should work to fix the tests.

Author: KorinaGraber

source/components/inputs/signature/signature.tests.ts

@@ -93,8 +93,10 @@ describe('SignatureComponent', () => {
 		let canvas: ICanvasMock;
 
 		beforeEach(() => {
-			canvas = {
+			canvas = <any>{
 				jSignature: sinon.spy(),
+				addEventListener: () => null,
+				removeEventListener: () => null,
 			};
 			(signature as any)._canvas = canvas;
 			signature.rendering = true;