UMD build is not enabled on UNPKG

[nickserv]

Visiting https://unpkg.com/redux displays the CJS build (https://unpkg.com/redux@4.0.0/lib/redux.js) when the UMD build (https://unpkg.com/redux@4.0.0/dist/redux.js) should be displayed instead.

Please refer to the usage instructions at the bottom of https://unpkg.com/.

[timdorr]

We added an unpkg field in #3003. It just hasn't been published.