React 16.3 Compatibility; slight performance improvements

[lsadam0]

Hello qiaolb,

This PR:

• Removes string ref usage and introduces compatibility with React 16.3
• Moves method binding from the render body into the constructor, this helps to avoid unnecessary re-renders
• Only computes container style when the height or width props have changed. This also helps to avoid unnecessary rerenders
• When the height and width props have changed, the recalculation of container style is handled by react 16.3's new getDerivedStateFromProps method. This replaces componentWillReceiveProps.

Thanks for maintaining this library!

[mylesthedev]

@qiaolb any chance you can merge this?