Add require kaminari/core to kaminari-activerecord

So that when using sinatra-activerecord independently (say, for a Sinatra app),
you can load this gem just by `require 'sinatra/activerecord'`.

kaminari-activerecord/lib/kaminari/activerecord.rb

@@ -3,6 +3,7 @@
 require 'active_support/lazy_load_hooks'
 
 ActiveSupport.on_load :active_record do
+  require 'kaminari/core'
   require 'kaminari/activerecord/active_record_extension'
   ::ActiveRecord::Base.send :include, Kaminari::ActiveRecordExtension
 end