error running rake test in actionpack

I get this error when I try to rake test in actionpack:

(in /home/miles/c/rails/actionpack)
/home/miles/c/rails/activesupport/lib/active_support/dependencies.rb:
212:in `require': no such file to load -- rack/mount (LoadError)
  from /home/miles/c/rails/activesupport/lib/active_support/
dependencies.rb:212:in `require'
  from /home/miles/c/rails/activesupport/lib/active_support/
dependencies.rb:198:in `load_dependency'
  from /home/miles/c/rails/activesupport/lib/active_support/
dependencies.rb:563:in `new_constants_in'
  from /home/miles/c/rails/activesupport/lib/active_support/
dependencies.rb:198:in `load_dependency'
  from /home/miles/c/rails/activesupport/lib/active_support/
dependencies.rb:212:in `require'
  from /home/miles/c/rails/actionpack/lib/action_dispatch/routing/
route_set.rb:1

Adding a clause like this:

group :test do
  gem "rack-mount"
end

To the Gemfile seems to fix it. Is that the right solution or am I
missing something?

bundle update should make the tests work again.

That did it. Thanks.