Check for `Element` prevents correctly working under jsdom

[carsonreinke]

Under a jsdom environment Element is not an Object.

if (typeof Element === 'undefined' || !(Element instanceof Object)) {
    return;
}

https://github.com/que-etc/resize-observer-polyfill/blob/master/src/ResizeObserverSPI.js#L76

Probably simply just removing the later condition unless there was some reason for that.

Related to #50.