Ensure rake integration is enabled when rescue_rake_exceptions is true

[Nitrodist]

An LoadError exception is now raised when the client is configured to
rescue from rake exceptions and airbrake/rake_handler has not been
loaded.

Fixes #292

[alif]

Can you confirm that cucumber tests pass? I ask this because many times people don't think to run them locally.

[Nitrodist]

Works for me:

 ~/airbrake <ensure-rake-integration> be rake cucumber
/Users/mcampbell/.rbenv/versions/2.0.0-p0/bin/ruby -S bundle exec cucumber --tags ~@wip features/rake.feature
Feature: Use the Gem to catch errors in a Rake application

  Background:                        # features/rake.feature:3
    Given I've prepared the Rakefile # features/step_definitions/rake_steps.rb:1

  Scenario: Ignoring exceptions                  # features/rake.feature:6
    When I run rake with airbrake ignored        # features/step_definitions/rake_steps.rb:7
    Then Airbrake should not catch the exception # features/step_definitions/rake_steps.rb:12

  Scenario: Catching exceptions in Rake      # features/rake.feature:10
    When I run rake with airbrake            # features/step_definitions/rake_steps.rb:7
    Then Airbrake should catch the exception # features/step_definitions/rake_steps.rb:16

  Scenario: Falling back to default handler before Airbrake is configured # features/rake.feature:14
    When I run rake with airbrake not yet configured                      # features/step_definitions/rake_steps.rb:7
    Then Airbrake should not catch the exception                          # features/step_definitions/rake_steps.rb:12

  Scenario: Disabling Rake exception catcher     # features/rake.feature:18
    When I run rake with airbrake disabled       # features/step_definitions/rake_steps.rb:7
    Then Airbrake should not catch the exception # features/step_definitions/rake_steps.rb:12

  Scenario: Autodetect, running from terminal              # features/rake.feature:22
    When I run rake with airbrake autodetect from terminal # features/step_definitions/rake_steps.rb:7
    Then Airbrake should not catch the exception           # features/step_definitions/rake_steps.rb:12

  Scenario: Autodetect, not running from terminal              # features/rake.feature:26
    When I run rake with airbrake autodetect not from terminal # features/step_definitions/rake_steps.rb:7
    Then Airbrake should catch the exception                   # features/step_definitions/rake_steps.rb:16

6 scenarios (6 passed)
18 steps (18 passed)
0m30.083s

[Nitrodist]

@alif the command only did it for that one feature thing, though... not sure how to get the rest of the features to run.