added explicit require for kaminari

[hansondr]

This fixes the following error:

undefined method `page_url_for' for class `Kaminari::Helpers::Tag'

which occurs when trying to alias :page_url_for before kaminari has been loaded simple_admin/kaminari:15

Reproduction

Version Info

Rails 4.1.1
Ruby 2.1.2p95

Console Commands

rails new simple_admin_test
cd simple_admin_test
echo "gem 'simple_admin', git: 'https://github.com/jeffrafter/simple_admin.git' " >> Gemfile
bundle install
rails g simple_admin:install
rails s

open browser to localhost:3000/admin expect to see the following: