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The dbm library which is part of Ruby stdlib is not installed if it doesn't find a dbm implementation (such as gdbm) on the computer.
I was unable to require 'yaml/dbm' on my Homebrew Ruby installation, so I uninstalled it, installed gdbm, and reinstalled Ruby, and the problem was fixed.
See this related issue by the developer of the Psych YAML engine (although on a different project).
We do this for Python, seems reasonable.
Fixed in bf3dfa9 - thanks for the suggestion.