Removed old syntax for React refs for React 16 support

[snaerth]

In React 16 there is a new syntax for accessing element through native React refs.

Set component ref

Old syntax: ref="slider"
New syntax: ref={r = (this.slider = r)}

Access ref within class

Old syntax: this.refs.slider
New syntax: this.slider

https://reactjs.org/docs/refs-and-the-dom.html

[axelg12]

This fixes #120 and I don't see any reason for not merging this. Any thoughts @mpowaga ?

[snaerth]

@mpowaga Could you a least tell us if you are willing to use this PR

[johnste]

The fat arrow syntax broke our build (or rather, uglify didn't understand it). Not saying it should be fixed, but it might be of interest.

[mpowaga]

Thanks for pointing this out @johnste. I'll fix it soon.