Since rake 11.x was released, rake tasks are failing with the following:

NoMethodError: undefined method `last_comment' for #<Rake::Application:0x00000000cc8350>
/usr/local/rvm/gems/ruby-2.1.5@test_develop-1/gems/rubocop-0.35.1/lib/rubocop/rake_task.rb:23:in `initialize'
/var/lib/workspace/workspace/test_develop/database/postgresql/ruby/2.1/slave/fast/lib/tasks/rubocop.rake:5:in `new'
/var/lib/workspace/workspace/test_develop/database/postgresql/ruby/2.1/slave/fast/lib/tasks/rubocop.rake:5:in `<top (required)>'
/usr/local/rvm/gems/ruby-2.1.5@test_develop-1/gems/activesupport-4.1.14.2/lib/active_support/dependencies.rb:241:in `load'
/usr/local/rvm/gems/ruby-2.1.5@test_develop-1/gems/activesupport-4.1.14.2/lib/active_support/dependencies.rb:241:in `block in load'
/usr/local/rvm/gems/ruby-2.1.5@test_develop-1/gems/activesupport-4.1.14.2/lib/active_support/dependencies.rb:232:in `load_dependency'
/usr/local/rvm/gems/ruby-2.1.5@test_develop-1/gems/activesupport-4.1.14.2/lib/active_support/dependencies.rb:241:in `load'
/usr/local/rvm/gems/ruby-2.1.5@test_develop-1/gems/railties-4.1.14.2/lib/rails/engine.rb:654:in `block in run_tasks_blocks'
/usr/local/rvm/gems/ruby-2.1.5@test_develop-1/gems/railties-4.1.14.2/lib/rails/engine.rb:654:in `each'
/usr/local/rvm/gems/ruby-2.1.5@test_develop-1/gems/railties-4.1.14.2/lib/rails/engine.rb:654:in `run_tasks_blocks'
/usr/local/rvm/gems/ruby-2.1.5@test_develop-1/gems/railties-4.1.14.2/lib/rails/application.rb:400:in `run_tasks_blocks'
/usr/local/rvm/gems/ruby-2.1.5@test_develop-1/gems/railties-4.1.14.2/lib/rails/engine.rb:449:in `load_tasks'
/usr/local/rvm/gems/ruby-2.1.5@test_develop-1/gems/railties-4.1.14.2/lib/rails/railtie.rb:194:in `public_send'
/usr/local/rvm/gems/ruby-2.1.5@test_develop-1/gems/railties-4.1.14.2/lib/rails/railtie.rb:194:in `method_missing'
/var/lib/workspace/workspace/test_develop/database/postgresql/ruby/2.1/slave/fast/Rakefile:7:in `<top (required)>'
/usr/local/rvm/gems/ruby-2.1.5@test_develop-1/bin/ruby_executable_hooks:15:in `eval'
/usr/local/rvm/gems/ruby-2.1.5@test_develop-1/bin/ruby_executable_hooks:15:in `<main>'

This is a bug in rubocop to be fixed in https://github.com/bbatsov/rubocop/pull/2931. In the meantime and for stable branches, the version of rake should probably be pinned to avoid updating rubocop.