[backend] Make 'Mail Methods' link highlight as the active link when at mail_methods/edit

Fixes #2961
Upgrades Kaminari to 0.14.1

* Removes default_per_page method which does not work to upgrade
  Kaminari. Similar to
  https://github.com/spree/spree/commit/a1f3f21ef1abb1fdbdcbd7ac8fb50e0e704f3c81

  Fixes #3488
[api] Use Ruby 1.8 syntax in spec/models/order_spec.rb