Update index.js

[jkomyno]

• removed useless function invokation
• switched from old style propTypes declaration to the static version. This makes all the code related to the component reside inside its class declaration, thus enhancing encapsulation and readability

[sonnylazuardi]

I think we still need to pass this reference
either this

this.startAnimation(this.changeIndex.bind(this));

or

changeIndex = () => {
  ...
  this.startAnimation(this.changeIndex);
}