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

Update ruby-build -> 20181207

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

(cherry picked from commit 6203957bd16a9b2c309ee3dd38118e2b4611cb0e)

Upgrade ruby_build to 20181019

Update ruby-build -> 20181018

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

Fix #69 - Filter out 'ruby-' in .ruby-version file

Newer versions of Rails is pre-baked to have `ruby-x.x.x` in the `.ruby-version` file.

This breaks the legacy support as the whole contents of the file is used as the ruby version.

This PR adds a filtering when getting the legacy version from the file (remove `ruby-` prefix if it exists).

Fix patch relative path reference

`get_absolute_path` is called with the patch file path as the first
argument.

This commit fixes the relative path reference in the function to use
the value of the first argument passed into it.

Update ruby-build

This ruby-build update comes with TruffleRuby support

Update doc: use `mv -T` to move rbenv rubies to asdf rubies

This allows `~/.rbenv/versions` to rename to `~/.asdf/installs/ruby` instead of ending up with `~/.asdf/installs/ruby/versions`.

use available CPU cores when compiling (fixes #42)

allow to apply custom patches during install (fixes #15)

Add template to mktemp call so it will work on OSX

Make default-gems install command match current Ruby version

https://github.com/asdf-vm/asdf-ruby/issues/31

Add support for a default-gems file

https://github.com/asdf-vm/asdf-ruby/issues/31

use https to download ruby source