URI.open via Kernel#open is deprecated

[estolfo]

Hi!
Ruby 3.0 has removed support for URI.open via Kernel#open, making these lines fail:

• reader.rb#L121
• validator.rb#L596

I was going to open a PR to change those lines but I noticed in your CI that you run your tests against ruby versions back to 1.9.
It seems that URI.open was made public in ruby 2.4. I guess the two options would be to drop support for ruby < 2.4 or use URI.send(:open, ...), which should work for all versions of ruby that you support (I have not verified this yet).

I'm happy to open a pull request and update your test matrix, just let me know what you'd like to do!
Thanks!