move deprecated RMagick.rb file

Not sure how people will feel about this, but a potential fix for https://github.com/gemhome/rmagick/issues/62
So, I started writing that this file can't be required, but saw you're adding `deprecated` to the require path below.  This is super surprising.  Would you mind putting a comment here that we're adding this file to the require path? Or maybe a README in the deprecated dir that says this?

Anything we can do you prevent future surprise would be great.

Also, I think it would be better to avoid require_relative and use `require File.expand_path(File.join('..','../lib/rmagick_internal'), __FILE__)`