Update gulp-* modules to latest versions

Update to npm@5

Upgrade to React 15.0.1

Use ES2015 arrow functions.

Fix touch events: bind handler functions to this.

Upgrade to React 15 RC2; switch back to returning null from GameEndOverlay.

Use a stateless function component for the GameEndOverlay.

Upgrade to React 0.14 RC1

Set key properties on all child components in arrays.

Don't use classSet; use React minified version without addons.

Upgrade to React 0.13 RC1; use ES6 classes.

Upgrade to React 0.12; use Babel instead of react-tools.

Add building and running instructions to readme

Add build gulp task that does not watch for changes.

Make Try again button work on touch devices.