Add create-react-class dependency

It was added in commit https://github.com/YouCanBookMe/react-datetime/commit/84755ceb54dd6f5fe7682912336b243e4b7dc59b but then remove in this commit https://github.com/YouCanBookMe/react-datetime/commit/752fa8b2aee6385115c86d74e8f641de48982323 for no obvious reason. Apparently it's causing some problems for people (https://github.com/YouCanBookMe/react-datetime/issues/399) so adding it back.