don't require rails and check for rails defined

[TSMMark]

Hi, this gem is awesome! So awesome that I wanted to use it in vanilla Ruby projects as well, and not only Rails projects.

This PR resolves all of the issues I faced when running this outside of a Rails-app environment.

Thank you!

[TSMMark]

Damn, so some other gems required by this gem also define a Rails module... E.g. https://github.com/rails/rails-html-sanitizer/blob/55818711cce9e7d7185a116abe7b0cada7414785/lib/rails-html-sanitizer.rb#L6-L7

This is really unfortunate because many gems check to see if we're in a "Rails app" by checking defined?(::Rails). So defining a module Rails just to nest modules & classes under it makes this check meaningless and it always seems like the codebase is a "Rails app". I'm going to try something else like monkeypatching defined?(::Rails) to always return false or something.

EDIT: defined? is an operator and not a method — so monkeypatching it won't work. Any suggestions?

[TSMMark]

Alright so I was able to get this working properly by forking 3 other gems and fixing their defined?(Rails) checks just like this one.

E.g. defined?(::Rails::Engine), defined?(::Rails::Railtie) etc.

[rafaelfranca]

This gem needs to continue being tested with Rails, so this can't be removed.

[rafaelfranca]

I don't think this change should be made like this. We should be configuring the application path, and using the Railtie we can set it to Rails.root

[TSMMark]

Rails.root is not a method in my case. Would you prefer just checking for the method

Suggested change

	end if defined?(Rails.root) && Rails.root
	end if defined?(Rails.root)

[rafaelfranca]

No. I'd prefer if this class didn't call Rails.root at all, and instead had a application_root configuration that could be set by this gem Railtie to ExceptionMapper.application_root = Rails.root