Turn on `--enable-shared` by default for all supported MRI Rubies

Some gems need the host ruby to have shared library enabled. Currently,
we're not aware of any potential downsides to having this enabled by
default.

> Shared libraries are libraries that are loaded by programs when they
> start. When a shared library is installed properly, all programs that
> start afterwards automatically use the new shared library.
> [...] if you have a program that needs a shared library (in this case ruby
> libruby.so.2.2.0 or similar) the program will fail if it doesn't exist.

Correct a typo in ruby-build

- Tweak release notes in editor before submitting
- Create annotated git tag
- Publish GitHub release by default instead of draft
- Remove Homebrew publish steps - now handled by Actions

Add definition for Ruby 2.7.0-preview2

Ruby 2.7.0-preview2 has been released.
https://www.ruby-lang.org/en/news/2019/10/22/ruby-2-7-0-preview2-released/

Add example of mirrored URL.

Add definition for Ruby 2.4.9.

Look for Homebrew openssl@1.1

Add Ruby 2.5.7

Add Ruby 2.4.8

Add ruby 2.6.5

ruby-build formula depends on openssl@1.1, let’s look for that version

Add TruffleRuby 19.2.0.1

Add definition for Ruby 2.5.6 and 2.4.7

Add definition for Ruby 2.6.4

These Rubies has been released.

- https://www.ruby-lang.org/en/news/2019/08/28/ruby-2-5-6-released/
- https://www.ruby-lang.org/en/news/2019/08/28/ruby-2-4-7-released/

Ruby 2.6.4 has been released.
https://www.ruby-lang.org/en/news/2019/08/28/ruby-2-6-4-released/

Add TruffleRuby 19.2.0

Add definition for JRuby 9.2.8.0

Closes #1335.

JRuby 9.2.8.0 has been released.
https://www.jruby.org/2019/08/12/jruby-9-2-8-0.html

Add TruffleRuby 19.1.0