Check that Railtie is defined before requiring verdict/railtie

I'd like to use verdict with pre-Railtie Rails, specifically this forked [github/rails](https://github.com/github/rails) version. However, verdict requires a Railtie if `Rails` is defined. Would it be possible to check for `Rails::Railtie` explicitly instead to determine whether `verdict/railtie` should be `require`d?

cc @alindeman @grantr