Fix create-react-class and prop-types references in UMD build

[banderson]

@TrevorBurnham I found the issue with the react-create-class dependency, it turns out we had to update the browserify config to point to the global before we build the UMD. This gets things working again for me locally.

[TrevorBurnham]

This could function as a workaround for our internal build issue, but at the expense of inducing a deprecation warning in React 15.5 and breaking the project entirely in React 16. Surely there must be a way to define a react-create-class module that maps to React.createClass as part of our React < 15.5 build, rather than foregoing the module entirely?

[TrevorBurnham]

@banderson react-select has significantly changed their build process since we last looked at this. Those changes will come in to react-select-plus once #96 is merged. Let's reassess after that's merged.