Test showing World is inconsistently available in Around hook

As @mattwynne requested on #52.

Test: `cd examples/hooks; bundle exec cucumber features`

Results on my machine:
```
I am a NilClass
Feature: Hooks
  Scenario

  Scenario: First scenario                 # features/hooks.feature:4
    Given the around hook has run          # features/step_definitions/hook_steps.rb:1
    Then the world should be a CustomWorld # features/step_definitions/hook_steps.rb:5

      expected: "CustomWorld"
           got: "NilClass"

      (compared using ==)
       (RSpec::Expectations::ExpectationNotMetError)
      ./features/support/env.rb:12:in `call'
      ./features/support/env.rb:12:in `block in <top (required)>'
      features/hooks.feature:6:in `Then the world should be a CustomWorld'

  Scenario: Second scenario                # features/hooks.feature:8
    Given the around hook has run          # features/step_definitions/hook_steps.rb:1
      I am a CustomWorld
    Then the world should be a CustomWorld # features/step_definitions/hook_steps.rb:5

  Scenario: Third scenario                 # features/hooks.feature:12
    Given the around hook has run          # features/step_definitions/hook_steps.rb:1
      I am a CustomWorld
    Then the world should be a CustomWorld # features/step_definitions/hook_steps.rb:5

Failing Scenarios:
cucumber features/hooks.feature:4 # Scenario: First scenario
```