no such file to load -- ZenTest but I have already installed ZenTest

$ rake spec cucumber --trace (in /home/soroush/projects/mediapilot) ** Invoke spec (first_time) ** Invoke db:test:prepare (first_time) ** Invoke db:abort_if_pending_migrations (first_time) ** Invoke environment (first_time) ** Execute environment ** Execute db:abort_if_pending_migrations ** Execute db:test:prepare ** Invoke db:test:load (first_time) ** Invoke db:test:purge (first_time) ** Invoke environment ** Execute db:test:purge ** Execute db:test:load ** Invoke db:schema:load (first_time) ** Invoke environment ** Execute db:schema:load ** Execute spec /home/soroush/.gem/ruby/1.8/gems/rails-2.3.5/lib/rails/gem_dependency.rb:119:Warning: Gem::Dependency#version_requirements is deprecated and will be removed on or after August 2010. Use #requirement no such file to load -- ZenTest

Getting this problem when running rake spec cucumber. Problem is that ZenTest gem is already installed :S