Tell Travis to test against the current branch's latest commit

Fix output of `asdf list-all ruby` when downloading ruby-build

Fix install / list-all commands always re-download ruby-build

Update ruby-build version to ruby 2.7

Update ruby-build to v20191223

Update ruby-build version to v20191205

Update ruby-build version to 20191124

Fix `bin/list-all`

Also, use `grep -v` (-v: Invert match) to remove the `topaz-dev` from
the results.

Fixes #142.

Update asdf-ruby to use version 20191111

Update ruby-build to v20191102

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

Skip recompiling openssl for truffleruby

Update ruby-build to v20191024

Adds Ruby 2.7.0-preview2

Update ruby-build to v20191004

This adds Ruby 2.4.9.

Update ruby-build to v20191002

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

Allow custom ruby-build versions/branches.

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`).

Update ruby-build to v20190828.

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

Change indentation

Update ruby-build to latest version

Use specific commits from ruby-build. Allow Ruby 2.6.3 Installation