Remove npm requirement

We don't really need npm requirement for this package.
Remove it from package requirement.

#40

Remove unnecessary state

#10

1. Remove unused React state
   We are not using the state at all, the state is exactly the same as props.
   Will be able remove the dependency of react-lifecycles-compat.
2. Start to support new version of React
   We have componentDidUpdate to write css property as React doesn't support it until 15.6.
   To prevent breaking changes, I just simply made it a named export, so if you are using a new version of React, you could do import { AspectRatio } from 'react-aspect-ratio'; which is much simpler
3. Remove extends from PureComponent
   The component takes children props, which will be different on each render already, extending from PureComponent doesn't have much performance benefit in this case.

Defensive against CSS minification

Discuss thread 53ec158

CSS minification could remove the parentheses which break the style.
Solution is to always add parentheses on render.

Thanks @huyz

Fix regression issue around nextProps

#4

Release 1.0.25

1. Bump react-lifecycles-compat for some browser fixes

https://github.com/reactjs/react-lifecycles-compat/blob/master/CHANGELOG.md

2. publish es module

Release 1.0.23

Support React 16.3, use getDerivedStateFromProps