Generic param default for MockStoreCreator

As of TypeScript 2.3, there is now [support for Generic parameter defaults](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-3.html). By setting a reasonable default type on `MockStoreCreator`, we can eliminate unneeded verbosity.

types/redux-mock-store/index.d.ts

@@ -1,7 +1,8 @@
-// Type definitions for Redux Mock Store 0.0
+// Type definitions for Redux Mock Store 0.0.1
 // Project: https://github.com/arnaudbenard/redux-mock-store
 // Definitions by: Marian Palkus <https://github.com/MarianPalkus>, Cap3 <http://www.cap3.de>
 // Definitions: https://github.com/borisyankov/DefinitelyTyped
+// TypeScript Version: 2.3
 
 import * as Redux from 'redux';
 
@@ -10,7 +11,7 @@ export interface MockStore<T> extends Redux.Store<T> {
     clearActions(): void;
 }
 
-export type MockStoreCreator<T> = (state?: T) => MockStore<T>;
+export type MockStoreCreator<T = {}> = (state?: T) => MockStore<T>;
 
 declare function createMockStore<T>(middlewares?: Redux.Middleware[]): MockStoreCreator<T>;