Accessing PropTypes via the main React package is deprecated

[valpet]

Found in console log:

Warning: Accessing PropTypes via the main React package is deprecated, and will be removed in React v16.0. Use the latest available v15.* prop-types package from npm instead. For info on usage, compatibility, migration and more, see https://fb.me/prop-types-docs

[valpet]

PR #36 seems to address this issue

[hon2a]

This blocks update to React 16.