On OS/X Sierra, after recently running a brew update I started receiving the error message Sorry, you can't use byebug without Readline when trying to run some rake tasks in my ruby project folder. I observed this in projects and gems that include byebug or pry in their Gemfile or gem.spec. I've found in my googling that many begin encountering this error message after running a brew update but there are other triggering conditions as well.
>> rake aws:show_config
WARN: Unresolved specs during Gem::Specification.reset:
mime-types (>= 0)
WARN: Clearing out unresolved specs.
Please report a bug if this causes problems.
Sorry, you can't use byebug without Readline. To solve this, you need to