require will_paginate

When you only add the gem `will_paginate-bootstrap` in the `Gemfile` of a Rails application, `will_paginate` itself won't be required. You have to require it yourself or explicitly add `will_paginate` in the Gemfile. Otherwise you get errors like this:

    undefined method `paginate' for #<ActiveRecord::Relation::XXX>

This patch always requires will_paginate.