rename intial probe action from `init` to `probe`

[fxck]

The first time to probe for an initial state if you did not provide one, the second time when Store is instantiated

https://gitter.im/ngrx/store?at=580662f3759f6d2436c41a32

[MikeRyanDev]

Looks like the probing may not be necessary soon. See ReactiveX/rxjs#2050

[JimLynchCodes]

Hi , sorry dude, but seriously firing"init" twice makes no sense at all and is extremely confusing/ frightening/ worrying for developers (it was for me at least).

Please change one "init" to "probe" and decide later if you want to remove probe (who came up with the name probe anyway lol).

[MarkPieszak]

Looks like the RxJs issue was just merged the other day (24th), so we should be all set now. Do we need any changes to implement this? @MikeRyan52