Sidekiq/web: require active_support/core_ext/object to fix paging scheduled jobs

Paging scheduled jobs fails with

    NoMethodError at /sidekiq/scheduled
    undefined method `try' for nil:NilClass
    file: paginator.rb location: block in page line: 16

The Sidekiq web application has been mounted with

    # config.ru
    require 'sidekiq/web'
    :
    run Rack::URLMap.new(
      '/sidekiq' => Sidekiq::Web
    )

When accessing *http://localhost:9292/sidekiq/scheduled*, the error above occurs, because the Active Support's `#try` method is invoked at line 16 in *lib/sidekiq/paginator.rb* without requiring `active_support/core_ext/object` first.

This PR requires *active_support/core_ext/object* in *lib/sidekiq/core_ext.rb*, it does not handle *LoadError*. I am wondering why Sidekiq re-implements methods if loading Active Support fails? Wouldn't it be much easier for Sidekiq to just depend on Active Support?