Fix issues with Celluloid 0.17

[mperham]

e.g.

CLI#parse::with config file#test_0008_sets queues:
ArgumentError: No actors given to Tree to supervise.
    /Users/mike/.gem/ruby/2.2.1/gems/celluloid-supervision-0.20.0/lib/celluloid/supervision/container/behavior/tree.rb:23:in `block in <class:Tree>'
    /Users/mike/.gem/ruby/2.2.1/gems/celluloid-supervision-0.20.0/lib/celluloid/supervision/configuration.rb:37:in `instance_eval'
    /Users/mike/.gem/ruby/2.2.1/gems/celluloid-supervision-0.20.0/lib/celluloid/supervision/configuration.rb:37:in `initialize'
    /Users/mike/.gem/ruby/2.2.1/gems/celluloid-supervision-0.20.0/lib/celluloid/supervision/configuration.rb:10:in `new'
    /Users/mike/.gem/ruby/2.2.1/gems/celluloid-supervision-0.20.0/lib/celluloid/supervision/configuration.rb:10:in `define'
    /Users/mike/.gem/ruby/2.2.1/gems/celluloid-supervision-0.20.0/lib/celluloid/supervision/container.rb:11:in `define'
    /Users/mike/.gem/ruby/2.2.1/gems/celluloid-supervision-0.20.0/lib/celluloid/supervision/service.rb:13:in `define'
    /Users/mike/.gem/ruby/2.2.1/gems/celluloid-0.17.0/lib/celluloid/actor_system.rb:58:in `block in start'
    /Users/mike/.gem/ruby/2.2.1/gems/celluloid-0.17.0/lib/celluloid/actor_system.rb:68:in `within'
    /Users/mike/.gem/ruby/2.2.1/gems/celluloid-0.17.0/lib/celluloid/actor_system.rb:57:in `start'
    /Users/mike/.gem/ruby/2.2.1/gems/celluloid-0.17.0/lib/celluloid.rb:154:in `start'
    /Users/mike/.gem/ruby/2.2.1/gems/celluloid-0.17.0/lib/celluloid/autostart.rb:3:in `<top (required)>'
    /Users/mike/.gem/ruby/2.2.1/gems/activesupport-4.1.7/lib/active_support/dependencies.rb:247:in `require'
    /Users/mike/.gem/ruby/2.2.1/gems/activesupport-4.1.7/lib/active_support/dependencies.rb:247:in `block in require'
    /Users/mike/.gem/ruby/2.2.1/gems/activesupport-4.1.7/lib/active_support/dependencies.rb:232:in `load_dependency'
    /Users/mike/.gem/ruby/2.2.1/gems/activesupport-4.1.7/lib/active_support/dependencies.rb:247:in `require'
    /Users/mike/src/sidekiq/lib/sidekiq/cli.rb:163:in `load_celluloid'
    /Users/mike/src/sidekiq/lib/sidekiq/cli.rb:43:in `parse'
    /Users/mike/src/sidekiq/test/test_cli.rb:168:in `block (3 levels) in <class:TestCli>'