It took me some doing to come up with this solution.
RxSwift 4.1.x and Xcode 9.3 at first didn’t seem to play nice.
Playgrounds also suffered. Something of which this RayWenderlich book made use of. Something had changed from when I first got the book to two weeks later when I could no longer run ‘pod install’ or ‘pod update’ and have the Playgrounds work inside the Projects.
The key; Cocoapods had an update to 1.5.0, well the Projects in which the Playgrounds worked were installed using Cocoapods 1.3.1.
Running ‘pod _1.3.1_ install’ was still using 1.5.0 however. Ho hum. So much for the really simple gem command working.
Long story short, I removed Cocoapods 1.5.0, it went back to using 1.3.1 and then my playgrounds worked again.
In all this process I found Xcode 9.2 was a bit finicky with being able to run the playgrounds once Xcode 9.3 was installed.
Basically Xcode 9.3 uses Reactive Swift (RxSwift) better.
It can do this properly;
and not flag it as an Error.
Note: Xcode 9.2 seemed to work best with RxSwift 4.0.0.
So if your RxSwift isn’t working very well, and its 2018 April, you might consider switching cocoapods version to 1.3.1 for installing RxSwift in a project with a playground.