A better/standard approach to __proto__

Please consider to use ES standard `Object.getProottypeOf()` instead of quirk and non-standard yet `__proto__` which could also be removed from `Object.prototype` for security reasons and make zepto weaker.

Please also consider that even if TC39 and es-discuss recognize `__proto__` a mistake, they won't consider `Object.setPrototypeOf()` until this library will use directly the `__proto__` property.

Thanks in advance for, I hope and at least, thinking about these changes. No performance affected, nothing different but a better support for the past, the present, and the future of JS.