YAML is part of the Ruby 1.9 so,
If the NailGun server is started,
YAML can still be required if not using the NailGun client.
But it cannot be required if using the NailGun client.
Either the client is not setting the correct mode, the server doesn't honor the version required by the client or by the NailGun start command, or NailGun somehow interferes with the loading of the 1.9 modules.