Fix some specs on rails 4.1.0.beta1

Guys,

I rebased my [start branch of rails 4.1.0](https://github.com/maurogeorge/shoulda-matchers/tree/rails4.1) on the [shoulda matchers rails 4.1.0](https://github.com/thoughtbot/shoulda-matchers/tree/jd-add-4-1-beta).

I am confusing because the GH says it is have conflicts.

Right now we have only 1 spec broken on the rails 4.1.0.beta1.

@drapergeek after you change [this line](https://github.com/maurogeorge/shoulda-matchers/blob/feb773b87b4e64f21c373d6e145164eaeea1b6ee/lib/shoulda/matchers/assertion_error.rb#L10) I got this errors on the others Appraisals:

```
>> BUNDLE_GEMFILE=/Users/mauro/Desenvolvimento/web/shoulda-matchers/gemfiles/4.0.0.gemfile bundle exec /Users/mauro/.rvm/gems/ruby-2.0.0-p247@shoulda-matchers/bin/rake
/Users/mauro/Desenvolvimento/web/shoulda-matchers/lib/shoulda/matchers/assertion_error.rb:10:in `require': cannot load such file -- minitest (LoadError)
	from /Users/mauro/Desenvolvimento/web/shoulda-matchers/lib/shoulda/matchers/assertion_error.rb:10:in `<module:Matchers>'
	from /Users/mauro/Desenvolvimento/web/shoulda-matchers/lib/shoulda/matchers/assertion_error.rb:2:in `<module:Shoulda>'
	from /Users/mauro/Desenvolvimento/web/shoulda-matchers/lib/shoulda/matchers/assertion_error.rb:1:in `<top (required)>'
	from /Users/mauro/Desenvolvimento/web/shoulda-matchers/lib/shoulda/matchers.rb:2:in `require'
	from /Users/mauro/Desenvolvimento/web/shoulda-matchers/lib/shoulda/matchers.rb:2:in `<top (required)>'
	from /Users/mauro/Desenvolvimento/web/shoulda-matchers/lib/shoulda-matchers.rb:1:in `require'
	from /Users/mauro/Desenvolvimento/web/shoulda-matchers/lib/shoulda-matchers.rb:1:in `<top (required)>'
	from /Users/mauro/.rvm/gems/ruby-2.0.0-p247@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:72:in `require'
	from /Users/mauro/.rvm/gems/ruby-2.0.0-p247@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:72:in `block (2 levels) in require'
	from /Users/mauro/.rvm/gems/ruby-2.0.0-p247@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:70:in `each'
	from /Users/mauro/.rvm/gems/ruby-2.0.0-p247@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:70:in `block in require'
	from /Users/mauro/.rvm/gems/ruby-2.0.0-p247@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:59:in `each'
	from /Users/mauro/.rvm/gems/ruby-2.0.0-p247@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:59:in `require'
	from /Users/mauro/.rvm/gems/ruby-2.0.0-p247@global/gems/bundler-1.3.5/lib/bundler.rb:132:in `require'
	from /Users/mauro/Desenvolvimento/web/shoulda-matchers/tmp/aruba/testapp/config/application.rb:7:in `<top (required)>'
	from /Users/mauro/Desenvolvimento/web/shoulda-matchers/tmp/aruba/testapp/config/environment.rb:2:in `require'
	from /Users/mauro/Desenvolvimento/web/shoulda-matchers/tmp/aruba/testapp/config/environment.rb:2:in `<top (required)>'
	from /Users/mauro/Desenvolvimento/web/shoulda-matchers/spec/support/test_application.rb:63:in `require'
	from /Users/mauro/Desenvolvimento/web/shoulda-matchers/spec/support/test_application.rb:63:in `load_environment'
	from /Users/mauro/Desenvolvimento/web/shoulda-matchers/spec/support/test_application.rb:13:in `load'
	from /Users/mauro/Desenvolvimento/web/shoulda-matchers/spec/spec_helper.rb:5:in `<top (required)>'
	from /Users/mauro/Desenvolvimento/web/shoulda-matchers/spec/shoulda/matchers/action_controller/filter_param_matcher_spec.rb:1:in `require'
	from /Users/mauro/Desenvolvimento/web/shoulda-matchers/spec/shoulda/matchers/action_controller/filter_param_matcher_spec.rb:1:in `<top (required)>'
	from /Users/mauro/.rvm/gems/ruby-2.0.0-p247@global/gems/rspec-core-2.14.7/lib/rspec/core/configuration.rb:896:in `load'
	from /Users/mauro/.rvm/gems/ruby-2.0.0-p247@global/gems/rspec-core-2.14.7/lib/rspec/core/configuration.rb:896:in `block in load_spec_files'
	from /Users/mauro/.rvm/gems/ruby-2.0.0-p247@global/gems/rspec-core-2.14.7/lib/rspec/core/configuration.rb:896:in `each'
	from /Users/mauro/.rvm/gems/ruby-2.0.0-p247@global/gems/rspec-core-2.14.7/lib/rspec/core/configuration.rb:896:in `load_spec_files'
	from /Users/mauro/.rvm/gems/ruby-2.0.0-p247@global/gems/rspec-core-2.14.7/lib/rspec/core/command_line.rb:22:in `run'
	from /Users/mauro/.rvm/gems/ruby-2.0.0-p247@global/gems/rspec-core-2.14.7/lib/rspec/core/runner.rb:80:in `run'
	from /Users/mauro/.rvm/gems/ruby-2.0.0-p247@global/gems/rspec-core-2.14.7/lib/rspec/core/runner.rb:17:in `block in autorun'
/Users/mauro/.rvm/rubies/ruby-2.0.0-p247/bin/ruby -S rspec spec/shoulda/matchers/action_controller/filter_param_matcher_spec.rb spec/shoulda/matchers/action_controller/redirect_to_matcher_spec.rb spec/shoulda/matchers/action_controller/render_template_matcher_spec.rb spec/shoulda/matchers/action_controller/render_with_layout_matcher_spec.rb spec/shoulda/matchers/action_controller/rescue_from_matcher_spec.rb spec/shoulda/matchers/action_controller/respond_with_matcher_spec.rb spec/shoulda/matchers/action_controller/route_matcher_spec.rb spec/shoulda/matchers/action_controller/route_params_spec.rb spec/shoulda/matchers/action_controller/set_session_matcher_spec.rb spec/shoulda/matchers/action_controller/set_the_flash_matcher_spec.rb spec/shoulda/matchers/active_model/allow_mass_assignment_of_matcher_spec.rb spec/shoulda/matchers/active_model/allow_value_matcher_spec.rb spec/shoulda/matchers/active_model/disallow_value_matcher_spec.rb spec/shoulda/matchers/active_model/ensure_exclusion_of_matcher_spec.rb spec/shoulda/matchers/active_model/ensure_inclusion_of_matcher_spec.rb spec/shoulda/matchers/active_model/ensure_length_of_matcher_spec.rb spec/shoulda/matchers/active_model/exception_message_finder_spec.rb spec/shoulda/matchers/active_model/have_secure_password_matcher_spec.rb spec/shoulda/matchers/active_model/helpers_spec.rb spec/shoulda/matchers/active_model/numericality_matchers/comparison_matcher_spec.rb spec/shoulda/matchers/active_model/numericality_matchers/odd_even_number_matcher_spec.rb spec/shoulda/matchers/active_model/numericality_matchers/only_integer_matcher_spec.rb spec/shoulda/matchers/active_model/validate_absence_of_matcher_spec.rb spec/shoulda/matchers/active_model/validate_acceptance_of_matcher_spec.rb spec/shoulda/matchers/active_model/validate_confirmation_of_matcher_spec.rb spec/shoulda/matchers/active_model/validate_numericality_of_matcher_spec.rb spec/shoulda/matchers/active_model/validate_presence_of_matcher_spec.rb spec/shoulda/matchers/active_model/validate_uniqueness_of_matcher_spec.rb spec/shoulda/matchers/active_model/validation_message_finder_spec.rb spec/shoulda/matchers/active_record/accept_nested_attributes_for_matcher_spec.rb spec/shoulda/matchers/active_record/association_matcher_spec.rb spec/shoulda/matchers/active_record/association_matchers/model_reflection_spec.rb spec/shoulda/matchers/active_record/have_db_column_matcher_spec.rb spec/shoulda/matchers/active_record/have_db_index_matcher_spec.rb spec/shoulda/matchers/active_record/have_readonly_attributes_matcher_spec.rb spec/shoulda/matchers/active_record/serialize_matcher_spec.rb spec/support/shared_examples/numerical_submatcher_spec.rb --color --format progress failed
```

I am running  `ruby 2.0.0p247 (2013-06-27 revision 41674) [x86_64-darwin10.8.0]` via rvm.

PS: I only was making the specs pass, when it is all green we can refactor.