Async code calls MyStore.listen() twice.

Hello, I've come across an issue where MyStore.listen() will get called twice in my unit tests/production code if I use async code within the store. Here is my store code, the apiResolver is just a light wrapper on top of axios.

```
tryRegister: function(registerObj) {
apiResolver.registerUser({
email: registerObj.email,
password: registerObj.password,
platform: 'app'
})
.then(function() {
this.setState({error: false, registered: true});
}.bind(this))
.catch(function(error) {
this.setState({error: {title: 'Registration Failure', message: error.data}});
}.bind(this));
}
```

Unit test code

```
it.only('if successfully registered, set state object registered to true', function(done) {

// Given
var apiResolverMock = {
registerUser: sinon.stub().returns(Promise.resolve(true))
};

resetRewires.push(rewiredAuth.__set__('apiResolver', apiResolverMock));

AuthStore.listen(function(state) {
console.log(state);
expect(state.registered).to.equal(true);
done();
});

expect(AuthStore.state.registered).to.equal(false);

// When
AuthActions.tryRegister({
email: 'email@example.com',
password: 'password'
});

});
```