Move React to peerDependencies

Currently the example code from README fails when React and react-kinetic are installed with npm:
"Uncaught Error: Invariant Violation: TestingComponent.render(): A valid ReactComponent must be returned. You may have returned undefined, an array or some other invalid object."

This is because react-kinetic installs its own copy of React and thus the component descriptor returned by it is not recognized by React. Putting React to `peerDependencies` fixes this.