OpenSSL::X509::Certificate#version= calls X509_set_version, and
that sets the version stored in the certificate.  However, the
version stored in certificate is one less than the actual
certificate version (https://www.openssl.org/docs/manmaster/man3/X509_set_version.html).
There are no version 4 certificates, and when using recent LibreSSL,
drb ssl tests all fail without this change.

By providing a 'changelog_uri' in the metadata of the gemspec a
'Changelog' link will be shown on https://rubygems.org/gems/drb
which makes it quick and easy for someone to check on the changes
introduced with a new version.

Details of this functionality can be found on https://guides.rubygems.org/specification-reference/

…rubygems

Provide a 'Changelog' link on rubygems.org/gems/drb

The book is out of print, and its page removed. So, I used archive.org's linking to an old page.

I also updated the Programming Ruby book link, to point at the right page.

[doc] Update links

Fix wrong certificate version

[Bugs #15711]

https://bugs.ruby-lang.org/issues/15711

The implementation of `ObjectSpace._id2ref` on JRuby (and TruffleRuby)
is very expensive, and on JRuby we normally do not have it enabled
because it impacts performance of the entire runtime.

This uses `ObjectSpace::WeakMap` instead of `ObjectSpace._id2ref` by
default and deprecates `DRb::WeakIdConv`.

Authored-by: Masatoshi SEKI <m_seki@mva.biglobe.ne.jp>

Noticed from Jeremy's recent PR, and he requested here #29 (comment)

InvokeMethod18Mixin is for supporting Ruby 1.6 or older but we don't
support Ruby 1.6 or older. So we can InvokeMethod18Mixin into
InvokeMethod.