The Cartfile documentation makes the assertion:
If no version requirement is given, any version of the dependency is allowed.
with t
Simply github "jakecraige/RGB"
will yell No tagged versions found for github "jakecraige/RGB"
Better is to use github "jakecraige/RGB" "master"
You may want to read Carthage Tutorial: Getting Started
branch name / tag name / commit name means “Use this specific git branch / tag / commit”. For example, you could specify master, or a commit has like 5c8a74a.