If run `bundle exec rake' without `rake' gem, the following error is occurred.
% bundle exec rake
/Users/watson/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/bundler-1.3.5/lib/bundler/rubygems_integration.rb:214:in `block in replace_gem': rake is not part of the bundle. Add it to Gemfile. (Gem::LoadError)
from /Users/watson/.rbenv/versions/2.0.0-p247/bin/rake:22:in `<main>'