Add necessary require of Minitest

When I called `app` in master branch, `NameError` happens.

```ruby
$ rails c
[1] pry(main)> app
NameError: uninitialized constant ActionDispatch::Integration::Session::Minitest
from /home/yaginuma/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/bundler/gems/rails-74c31ac5fe44/actionpack/lib/action_dispatch/testing/integration.rb:139:in `<class:Session>'
``` 
Probably it is thought that `Minitest` is necessary.