check for definition of Rails::VERSION::MAJOR

[staugaard]

Some gems will declare the Rails module even without having a dependency on the rails gem.

I'm looking at you rails-dom-testing.
https://github.com/rails/rails-dom-testing/blob/master/lib/rails/dom/testing/version.rb

So you can end up with a Rails module without a VERSION constant.

Hi James!

[jamesarosen]

Is defined?(Foo::Bar::Baz) legit across modern Rubies? If so, big +1 :)

[pschambacher]

I think that it is legit