Add returnsThis method to SinonStub

sinon/sinon.d.ts

@@ -92,6 +92,7 @@ declare module Sinon {
         resetBehavior(): void;
         returns(obj: any): SinonStub;
         returnsArg(index: number): SinonStub;
+        returnsThis(): SinonStub;
         throws(type?: string): SinonStub;
         throws(obj: any): SinonStub;
         callsArg(index: number): SinonStub;