* Add jruby-9.2.9.0
* Require Java 8+ for JRuby 9.2+

Adds Ruby 2.7.0-preview2

This adds Ruby 2.4.9.

This adds Ruby 2.4.8, 2.5.7 and 2.6.5, as well as TruffleRuby
19.2.0.1.

Users can set `ASDF_RUBY_BUILD_VERSION` to their preferred version/branch (e.g. `master` if they always want the very latest from `ruby-build`).

This includes MRI v2.6.4, v2.5.6 and v2.4.7.

Addressing #108 and given ruby-build's reluctance to cut timely releases as
stated therein and from the perspective of ruby-build users, move asdf-ruby to
pulling specific commits from ruby-build.

Adds support for ruby v2.4.6

https://github.com/rbenv/ruby-build/releases/tag/v20190401

This should allow installation of ruby 2.5.5 and truffleruby-1.0.0-rc14

https://github.com/rbenv/ruby-build/releases/tag/v20190320

It will allow to install Ruby 2.6.0
See https://github.com/rbenv/ruby-build/commit/9ca19c24d42fccdcbd4592f0736437f635e2d8a5

changes from previous version since 20181019:
- 20181106
  - Add 2.6.0-preview3
  - truffleruby-1.0.0-rc8, 1.0.0-rc9
- 20181207
  - Add 2.6.0-rc1
  - Add jruby-9.2.1.0, 9.2.2.0, 9.2.3.0, 9.2.4.0, 9.2.4.1, 9.2.5.0
  - Add truffleruby-1.0.0-rc10

This update contains recent bumps for reported security issues on MRI
Ruby.

This ruby-build update comes with TruffleRuby support