Update promise example to work with Mocha

Author: jmccure

Diff for: README.md

@@ -111,17 +111,15 @@ You can also use promises:
 
 ```js
 describe('GET /users', function() {
-  it('responds with json', function(done) {
+  it('responds with json', function() {
     return request(app)
       .get('/users')
       .set('Accept', 'application/json')
       .expect('Content-Type', /json/)
       .expect(200)
       .then(response => {
           assert(response.body.email, 'foo@bar.com')
-          done();
       })
-      .catch(err => done(err))
   });
 });
 ```