Use new stop() method on matrix-mock-request

To finish all pending flushes between tests. This stops the unit
tests from hanging on node 11 when run in certain combinations.

Requires https://github.com/matrix-org/matrix-mock-request/pull/6
(so will need a release of matrix-mock-request before merging)

spec/integ/matrix-client-opts.spec.js

@@ -64,6 +64,7 @@ describe("MatrixClient opts", function() {
 
     afterEach(function() {
         httpBackend.verifyNoOutstandingExpectation();
+        return httpBackend.stop();
     });
 
     describe("without opts.store", function() {