Add require kaminari/core to kaminari-activerecord

[yhara]

I tried to use kaminari-activerecord in my Sinatra app, and noticed that two require's are actually needed:

Gemfile:

gem 'kaminari-activerecord'

app.rb:

require 'kaminari/core'  # error is raised without this
require 'kaminari/activerecord'

This PR allows you to load this gem just by:

require 'kaminari/activerecord'

[amatsuda]

@yhara Thank you!