Fix JRuby install on systems with non-BSD `sed`

There doesn't seem to be a way to construct a sed invocation for
in-place editing that is compatible with both BSD and GNU sed.

http://stackoverflow.com/q/5694228/11687

Fixes #475
Thorougly test JRuby install process
Merge branch 'jruby-sed'

Closes #476
ruby-build 20131220.1
preparation of Ruby 2.1
use official ruby repository
Merge pull request #477 from hsbt/heroku-support

Added definitions of Heroku's backports.
added md5
Merge pull request #479 from hsbt/release-2-1-0

Definitions of Ruby 2.1
Unify tense in CHANGELOG
Rename 2.1.0 -> 2.1.0-p0
ruby-build 20131225
fix some wrong name
Revert "Rename 2.1.0 -> 2.1.0-p0"

This reverts commit 36b5f33e53dc928d43eb7bcc6cba5f7a27e43fbb.

See https://twitter.com/hsbt/status/415887230318567424.
ruby-build 20131225.1
Add Rubinius 2.2.2