Use react-transition-group from npm

[gaearon]

Note: I have not tested this yet.

This should point react-addons-transition-group to API-compatible versions of react-transition-group package. This lets it keep working in observable future. So we don't need to deprecate it officially (even though we will keep the npm package deprecated and suggest people to migrate to new package explicitly).

cc @jquense

[flarnie]

If/when I have time I can come back to this and manually test it. I think we could use the 'browserify' fixture and require and use it there to manually test.

[gaearon]

I'm going to assume it works—I'll do another round of testing anyway as I complete preparing this, and I'd rather test them all at once.

[jquense]

Looks like it should work to me...i don't think I changed the exports at all 👍