ruby mongodb driver 1.8.1 not connecting (legacy connection) to db on Debian GNU/Linux (1.8.0 and 1.7.x could)

code.rb

require 'mongo'
con = Mongo::Connection.new('127.0.0.1', 27017)

Gemfile

source :rubygems

#gem 'mongo', '1.7.1'
#gem 'mongo', '1.8.0'
gem 'mongo', '1.8.1'

out.txt

jmettraux@tachibana ~/tmp/mongo 立 bundle exec ruby code.rb 
      ** Notice: The native BSON extension was not loaded. **

      For optimal performance, use of the BSON extension is recommended.

      To enable the extension make sure ENV['BSON_EXT_DISABLED'] is not set
      and run the following command:

        gem install bson_ext

      If you continue to receive this message after installing, make sure that
      the bson_ext gem is in your load path.

/home/jmettraux/.rvm/gems/ruby-1.9.3-p194/gems/mongo-1.8.1/lib/mongo/mongo_client.rb:479:in `connect': Failed to connect to a master node at 127.0.0.1:27017 (Mongo::ConnectionFailure)
  from /home/jmettraux/.rvm/gems/ruby-1.9.3-p194/gems/mongo-1.8.1/lib/mongo/mongo_client.rb:631:in `setup'
	from /home/jmettraux/.rvm/gems/ruby-1.9.3-p194/gems/mongo-1.8.1/lib/mongo/mongo_client.rb:147:in `initialize'
	from /home/jmettraux/.rvm/gems/ruby-1.9.3-p194/gems/mongo-1.8.1/lib/mongo/legacy.rb:56:in `initialize'
	from code.rb:3:in `new'
	from code.rb:3:in `<main>'