How to Fix Ruby 1.9.3 "It seems your ruby installation is missing psych" Error

If you see this:

/usr/local/lib/ruby/1.9.1/yaml.rb:56: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.
Latest version currently installed. Aborting.

First do this: 

sudo apt-get install libyaml-dev

Then rebuild/reinstall your Ruby.