React-big-calendar: Use 'create-react-class' package instead of React.createClass

[MichaelDeBoey]

React.createClass has been deprecated and extracted to it's own package since React v15.5.0.

The following warning is displayed if that version of React is used:
Warning: Calendar: React.createClass is deprecated and will be removed in version 16. Use plain JavaScript classes instead. If you're not yet ready to migrate, create-react-class is available on npm as a drop-in replacement.

See Migrating from React.createClass to fix this.

This could be a temporary fix, but I think moving to ES6 classes is more appropriate :-)

[jquense]

PR's welcome!

[MichaelDeBoey]

Will try to look at it this evening :-)