::Mongoid.default_session.disconnect is being used since v3.0.11 - see changes here. The Session#disconnect method was only implemented in Moped 1.2.0, but the Mongoid gemspec only depends on Moped ~> 1.1. This can cause issues when using Mongoid in Passenger or Unicorn. Manually specifying a dependency on Moped ~> 1.2 is a workaround for the issue.

Note that this has been fixed in master, but not yet in 3.0.0-stable.