added explicit require for kaminari

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](https://github.com/jeffrafter/simple_admin/blob/master/lib/simple_admin/kaminari.rb#L15)

### 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:
![kaminari_error](http://i.imgur.com/IZxq6MT.png)