gemでoutdatedしたら、あれれ。
~$ gem outdated /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/yaml.rb:5:in `<top (required)>': It seems your ruby installation is missing psych (for YAML output). To eliminate this warning, please install libyaml and reinstall your ruby. /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require': dlopen(/usr/local/lib/ruby/gems/2.4.0/gems/psych-2.2.4/lib/psych.bundle, 9): Library not loaded: /usr/local/opt/ruby/lib/libruby.2.4.0.dylib (LoadError) Referenced from: /usr/local/lib/ruby/gems/2.4.0/gems/psych-2.2.4/lib/psych.bundle Reason: image not found - /usr/local/lib/ruby/gems/2.4.0/gems/psych-2.2.4/lib/psych.bundle from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require' from /usr/local/lib/ruby/gems/2.4.0/gems/psych-2.2.4/lib/psych.rb:8:in `<top (required)>' from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require' from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require' from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/yaml.rb:6:in `<top (required)>' from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require' from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require' from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/rubygems.rb:700:in `load_yaml' from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/rubygems/config_file.rb:340:in `load_file' from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/rubygems/config_file.rb:202:in `initialize' from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/rubygems/gem_runner.rb:75:in `new' from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/rubygems/gem_runner.rb:75:in `do_configuration' from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/rubygems/gem_runner.rb:40:in `run' from /usr/local/opt/ruby/bin/gem:21:in `<main>'
ググってみたら、
Upgrading to Ruby 2.4.1_1 breaks gem command. · Issue #11636 · Homebrew/homebrew-core · GitHub
brew reinstall -s libyaml brew reinstall -s ruby
でも解消せず。さらに
rm -rf /usr/local/lib/ruby/gems/2.4.0/gems/psych-2.2.4 rm -rf /usr/local/lib/ruby/gems/2.4.0/extensions/x86_64-darwin-16/2.4.0/psych-2.2.4
これで gem outdated出来た。とりあえずめでたし。