Fix React deprecation warning - use 'prop-types' package

[inlinestyle]

Hi 👋 !
The latest versions of React emit a deprecation warning when React.PropTypes is used. This PR makes the recommended change to using the standalone prop-types package. It also adds prop-types as a peer dependency.

Let me know if any changes are needed!

Documentation: https://facebook.github.io/react/blog/2017/04/07/react-v15.5.0.html#migrating-from-react.proptypes.

[bebraw]

Thanks! Published as 0.6.0.

Note that I don't use the package actively anymore so let me know if you want to take over. I can set up rights etc.

[inlinestyle]

@bebraw sure! I'd be happy to be a maintainer or co-owner.

[bebraw]

@inlinestyle Ok, cool. I added you as a collaborator to the repository. You can also publish new versions of the package. I don't mind moving the package below your account either if you want (no strong opinion on that).

[inlinestyle]

@bebraw I'd say move it if you don't want to get @-mentioned anymore? Do you still want to review changes?

[bebraw]

@inlinestyle Ok, I'll move then. You have to remove your personal fork to enable the move. Good to go after that.

[inlinestyle]

@bebraw done!

[inlinestyle]

@bebraw thanks! I'll stop pinging you now :)