Add entry point for webpack v2

[mrtnbroder]

Just like for Redux, let's introduce an entry point for webpack v2.

[taion]

It's already there on next.

It turns out that this makes a broken config for anybody who's using webpack 2 but hasn't turned off module transpilation: relay-tools/react-router-relay#189, relay-tools/react-router-relay#192.

Given everything else that's going on, I'd rather not deal with the support load that comes from this.