Removed duplicated test.

`test_display_exception_details_cause_loop` is same as
`test_display_exception_details_cause`.

Fixed ruby#185

test/test_rake_application.rb

@@ -57,33 +57,6 @@ def test_display_exception_details_cause
     assert_match 'cause b', err
   end
 
-  def test_display_exception_details_cause_loop
-    skip 'Exception#cause not implemented' unless
-      Exception.method_defined? :cause
-
-    begin
-      begin
-        raise 'cause a'
-      rescue => a
-        begin
-          raise 'cause b'
-        rescue
-          raise a
-        end
-      end
-    rescue => ex
-    end
-
-    out, err = capture_io do
-      @app.display_error_message ex
-    end
-
-    assert_empty out
-
-    assert_match 'cause a', err
-    assert_match 'cause b', err
-  end
-
   def test_display_tasks
     @app.options.show_tasks = :tasks
     @app.options.show_task_pattern = //