Update to work with Ruby 2.4.0

[drewdeponte]

Doing so resulted in the following errors but I haven't had a chance to dig into them.

Failures:

  1) Togls::ToggleRegistry#feature returns the created toggler
     Failure/Error: @features[feature_id] = Marshal.dump(feature_data)
     
     RuntimeError:
       RSpec::Mocks::Double#marshal_dump returned same class instance
     # ./lib/togls/feature_repository_drivers/in_memory_driver.rb:17:in `dump'
     # ./lib/togls/feature_repository_drivers/in_memory_driver.rb:17:in `block in store'
     # ./lib/togls/feature_repository_drivers/in_memory_driver.rb:16:in `synchronize'
     # ./lib/togls/feature_repository_drivers/in_memory_driver.rb:16:in `store'
     # ./lib/togls/feature_repository.rb:20:in `block in store'
     # ./lib/togls/feature_repository.rb:19:in `each'
     # ./lib/togls/feature_repository.rb:19:in `store'
     # ./lib/togls/toggle_repository.rb:18:in `store'
     # ./lib/togls/toggle_registry.rb:23:in `feature'
     # ./spec/togls/toggle_registry_spec.rb:97:in `block (3 levels) in <top (required)>'
     # ./spec/spec_helper.rb:33:in `block (2 levels) in <top (required)>'

  2) Togls::ToggleRegistry#feature creates a feature toggle with the created feature
     Failure/Error: @features[feature_id] = Marshal.dump(feature_data)
     
     RuntimeError:
       RSpec::Mocks::Double#marshal_dump returned same class instance
     # ./lib/togls/feature_repository_drivers/in_memory_driver.rb:17:in `dump'
     # ./lib/togls/feature_repository_drivers/in_memory_driver.rb:17:in `block in store'
     # ./lib/togls/feature_repository_drivers/in_memory_driver.rb:16:in `synchronize'
     # ./lib/togls/feature_repository_drivers/in_memory_driver.rb:16:in `store'
     # ./lib/togls/feature_repository.rb:20:in `block in store'
     # ./lib/togls/feature_repository.rb:19:in `each'
     # ./lib/togls/feature_repository.rb:19:in `store'
     # ./lib/togls/toggle_repository.rb:18:in `store'
     # ./lib/togls/toggle_registry.rb:23:in `feature'
     # ./spec/togls/toggle_registry_spec.rb:63:in `block (3 levels) in <top (required)>'
     # ./spec/spec_helper.rb:33:in `block (2 levels) in <top (required)>'

Finished in 0.13267 seconds (files took 0.38276 seconds to load)
316 examples, 2 failures

Failed examples:

rspec ./spec/togls/toggle_registry_spec.rb:89 # Togls::ToggleRegistry#feature returns the created toggler
rspec ./spec/togls/toggle_registry_spec.rb:57 # Togls::ToggleRegistry#feature creates a feature toggle with the created feature