fix the deprecated formatter interface warning

Fixed the warning "The Nc formatter uses the deprecated formatter interface not supported directly by RSpec 3.  To continue to use this formatter you must install the `rspec-legacy_formatters` gem, which provides support for legacy formatters or upgrade the formatter to a compatible version.".

refer to: http://myronmars.to/n/dev-blog/2014/02/rspec-2-99-and-3-0-beta-2-have-been-released#rspeccore_300beta2 and http://rubydoc.info/gems/rspec-core/RSpec/Core/Formatters