Specify version for minitest-rails-capybara

This will make sure minitest-rails and minitest-rails-capybara versions
are compatible.
Make test work 7 days before the end of the month

This is nasty; there should be specific tests for each case: Only this
month, also next month, with tests for the borderline cases.
Upgrade to Rails 4.1

- Update dependencies on rails and minitest-rails
- Load 'rails/all' to ensure tests run in test environment