Requires celluloid before requiring sidekiq/fetch

Requires celluloid before requiring sidekiq/fetch to avoid this error: https://github.com/brainopia/sidekiq-limit_fetch/pull/5

    $ rspec spec/
    /Users/fred/.rvm/gems/ruby-2.0.0-p247/gems/sidekiq-2.12.4/lib/sidekiq/actor.rb:4:in `included': uninitialized constant Sidekiq::Actor::Celluloid (NameError)