Support for bs-platform 2.2.2+ / Reason 3.1.0

[ckknight]

As per https://reasonml.github.io/blog/2018/03/06/reason-3.1.0.html, Reason 3.1.0 support comes with bs-platform 2.2.2 and it would be nice to use that new syntax with the bsb-native fork.

[bsansouci]

Master's up to date, will make a release very soon :)