Add browserify 3.x support

`util.isBuffer(new Buffer(4))` currently returns false with core-util-is and browserify 3.x.

In short, `instanceof Buffer` doesn’t work to check if an object is a Buffer in browserify. In browserify, the Buffer constructor actually returns an instance of `Uint8Array` for performance reasons.

So, can we change this to use `Buffer.isBuffer` instead? That lets browserify use it’s own `Buffer.isBuffer` implementation. I believe that in node `Buffer.isBuffer` just does `instanceof Buffer` anyway.