Leverage assertion test helpers

Add JRuby 1.7.24

Checksum refactor

Bash 3 doesn't have associative arrays. Use variable indirection to
save various checksum algorithm support results. `printf -v` can save
the output to a variable, whose name is itself stored in a variable

- Remove unnecessary HAS_X_SUPPORT variables
- Merge conditional for unsetting mirror-url
- Memoize has_checksum_support function

Explicitly specify destination refspec

`git push <remote> HEAD` is documented to push current branch to
same-named branch on the remote

Leverage assert_success helper

rbenv ensures RBENV_ROOT is set for us

Clean up usage documentation

And separate test body into arrange-act-assert blocks via newlines

Update rubinius builds openssl from 1.0.1q to 1.0.2e

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