Fix bug where waitsFor has an optional failureMessage

It should be defined as optional for failureMessage according to the specifications.

jasmine/jasmine.d.ts

@@ -21,7 +21,7 @@ declare function expect(actual: any): jasmine.Matchers;
 declare function spyOn(object: any, method: string): jasmine.Spy;
 
 declare function runs(asyncMethod: Function): void;
-declare function waitsFor(latchMethod: () => boolean, failureMessage: string, timeout?: number): void;
+declare function waitsFor(latchMethod: () => boolean, failureMessage?: string, timeout?: number): void;
 declare function waits(timeout?: number): void;
 
 declare module jasmine {