I answered at SO, but replicate the answer here too.
As a workaround, you should now explicitly pass user credentials. It is also done in preparations to Couchbase Server 5, where authentication and security in general has been improved a lot.
require 'couchbase'
@client = Couchbase.new(:node_list => ['172.19.4.22'], :bucket => 'markets',
:username => 'markets')
@client.design_docs