Since I updated
Reactive Cocoa (RAC 5)
Reactive Cocoa / Reactive Swift
Some parts of the Obj-C API have been divided in another framework : ReactiveObjC.
I needed to install this framework to access these methods.
As stated in README (Objective-C and Swift section), those Objective-C API are splitted out to ReactiveObjC framework. You need to add https://github.com/ReactiveCocoa/ReactiveObjC as a submodule, link the framework, then import ReactiveObjC.
Please see the following discussion on the issue :