Upgrade immutable to 3.8.2 because it was re-licensed to MIT

In the 3.x branch of immutable-js, the latest patch (Oct 4, 2017) relicensed the software to MIT https://github.com/immutable-js/immutable-js/releases/tag/v3.8.2. It would be useful to my organization if you could upgrade your relay-compiler package dependency on immutable from `~3.7.6` to `^3.8.2`. It could be mutually beneficial, I think, because that license would then match relay's.