class Puma::Configuration

def rack_builder

fallback to our minimal version.
Load and use the normal Rack builder if we can, otherwise 
def rack_builder
  # Load bundler now if we can so that we can pickup rack from
  # a Gemfile
  if ENV.key? 'PUMA_BUNDLER_PRUNED'
    begin
      require 'bundler/setup'
    rescue LoadError
    end
  end
  begin
    require 'rack'
    require 'rack/builder'
  rescue LoadError
    # ok, use builtin version
    return Puma::Rack::Builder
  else
    return ::Rack::Builder
  end
end