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
Hmm, interesting. I was looking at some other codebases and it looks like rspec actually prefers `require_relative` because it's faster thanks to skipping the load path: https://github.com/rspec/rspec-support/blob/master/lib/rspec/support.rb#L10

Though Avdi Grimm [makes an argument against using it](https://rubytapas.dpdcart.com/subscriber/post?id=602). Looks like the issue with evaluated contexts is no longer a problem in more recent versions of ruby. He argues that we should just unshift the relevant directories onto the load path and then use a regular `require` without all the `__FILE__` joining business. I need to test, but I think there may be an issue with that, too, if two gems add to the load path and require the same filename.