Sorry, you can't use byebug without Readline. To solve this, you need to rebuild Ruby with Readline support. If using Ubuntu, try `sudo apt-get install libreadline-dev` and then reinstall your Ruby

いきなりこんなエラーが出るようになりました。

$ rails s
Sorry, you can't use byebug without Readline. To solve this, you need to
rebuild Ruby with Readline support. If using Ubuntu, try `sudo apt-get
install libreadline-dev` and then reinstall your Ruby.
/Users/myname/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:274:in `require': dlopen(/Users/myname/.rbenv/versions/2.3.0/lib/ruby/2.3.0/x86_64-darwin14/readline.bundle, 9): Library not loaded: /usr/local/opt/readline/lib/libreadline.6.dylib (LoadError)
Referenced from: /Users/myname/.rbenv/versions/2.3.0/lib/ruby/2.3.0/x86_64-darwin14/readline.bundle

どうやらbrew updateを走らせたことが原因らしいです。

ln -s /usr/local/opt/readline/lib/libreadline.dylib /usr/local/opt/readline/lib/libreadline.6.dylib

シンボリックリンクを張り直すことで戻りました。