`marathon add RecativeSwift` fails because there's no official v4 release yet

[NicholasTD07]

Command:

marathon add https://github.com/ReactiveCocoa/ReactiveSwift.git

Output:

   Updating packages...
   $ cd "/Users/nicholas.tian/proj/TTTTT/" && /usr/bin/env xcrun --sdk macosx swift package --version
   Apple Swift Package Manager - Swift 4.1.0 (swiftpm-14050)
   $ cd "/Users/nicholas.tian/.marathon/Packages/Generated/" && /usr/bin/env xcrun --sdk macosx swift package update
   Updating https://github.com/ReactiveCocoa/ReactiveSwift.git
   error: dependency graph is unresolvable; found these conflicting requirements:

   Dependencies:
       https://github.com/ReactiveCocoa/ReactiveSwift.git @ 4.0.0..<5.0.0
💥  Failed to update packages
👉  Make sure you have write permissions to the folder '/Users/nicholas.tian/.marathon/Packages/'

Problem:

1. marathon fails to update the package
2. marathon gives the irrelevant error message Make sure you have write permissions to the folder

Possible cause:

The latest release version of RecativeSwift is 4.0.0-rc.2.

https://github.com/ReactiveCocoa/ReactiveSwift/releases

[NicholasTD07]

Thanks for the work! I love this idea of an easy command line version of a playground.