bump dependancy to rc9

[andrewmclagan]

We are using without error immutable-rc9

Should we bump the peer dependancy

"immutable": "^3.8.1 || ^4.0.0-rc.1",

"immutable": "^3.8.1 || ^4.0.0-rc.9",

[Aghassi]

4.0 is out now https://github.com/immutable-js/immutable-js/releases/tag/v4.0.0