Add metadata for Rubinius 3.0 aka rbx-3.0.0.

add message when bzip2 is missing and package type is bz2

Fixes #870 

On CentOS 7 and RHEL 7 bzip2 is not installed by default. This
causes a confusing error from tar when attempting to untar the
package. While this does not solve the problem, it at least gives
the user a more helpful error. More details can be found in GitHub
issue #870

https://travis-ci.org/rbenv/ruby-build/builds/99257003

Add Rubinius 2.10

References af2ffeb6

Add mruby 1.2.0

Release:
https://github.com/mruby/mruby/releases/tag/1.2.0

Announcements:
http://www.mruby.org/releases/2015/11/18/mruby-1.2.0-released.html

Add rbx-2.6.0, rbx-2.7.0, rbx-2.8.0, rbx-2.9.0

Internally, ruby-build will want to set `--with-openssl-dir` if we try
to detect Homebrew's OpenSSL or build one for the OS X. However,
Rubinius doesn't support that configure flag. So, this translates
`--with-openssl-dir` to a combination of `--with-lib-dir` and
`--with-include-dir`.

[ci skip]

added definition of Ruby 2.3.0

Ruby 2.3.0 branch and trunk

Using hub to fork Homebrew and push a branch we've just created to this
fork will be rejected by GitHub because the push is coming from a
shallow repository. Ensure that Homebrew is unshallowed before that.

[ci skip]

Closes #847

added definition of 2.1.8 and 2.2.4

Added definition of 2.3.0-preview2

Provide completion of flags for install/uninstall