I did a little googling (instead of learning for my exam :P ) and I think the cause of the two consistently failing REE builds on Travis could be a gem that is overwriting the String::to_xs method with an incompatible version that uses a different signature, hence the error:
l if atom is requested
Failure/Error: get :public, :username => @user.username, :format => :atom
wrong number of arguments (1 for 0)
# /home/vagrant/.rvm/gems/ree-1.8.7-2012.02/gems/builder-3.0.0/lib/builder/xmlbase.rb:135:in `to_xs'
(to_xs returns a string that is escaped for use in xml)
Question is, why isn't this happening with ruby 1.9.2 ...
this was most probably caused by the rails_admin gem indirectly using hpricot which does some twisted stuff to the String class.
Upstream bug report here -> sferik/rails_admin#1087
also this -> hpricot/hpricot#53 (note, that hpricot is 'closed')