Allows for passing environment variables specifically to the "make install"
step of package building.

This can be helpful for providing a different DESTDIR= to a ruby install from
the --prefix that is used

GNU sed compatibility

GNU sed version 4.1.5 on CentOS 5.8 does not support the -E flag:
$ sed -E
sed: invalid option -- E

Believe it or not, this was causing the following error:

    command not found-build: line 370: build_package_jruby

Closes #368

Benefits:
- makes it easier to stub `rbenv-*` subcommands in tests
- speeds up subcommands because it skips the main `rbenv` executable

Caveats:
- users are no longer able to call `bin/rbenv-install` or
  `rbenv-uninstall` directly

Same approach taken in rbenv: sstephenson/rbenv@baf7656d

Add Rubinius 2.2.3

This reverts commit 36b5f33e.

See https://twitter.com/hsbt/status/415887230318567424.

Definitions of Ruby 2.1

Added definitions of Heroku's backports.

Closes #476

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

The default shebang for binstubs in a JRuby installation is
`#!/usr/bin/env jruby`. On a rbenv-managed JRuby, this will execute the
`jruby` rbenv shim. In a situation where a JRuby tool like `gem` is
executed from within a running rbenv by a rbenv plugin, (e.g.
rbenv-gemset) this can trigger another rbenv run to resolve the shim.
This can lead to endless recursion of rbenv calls.

Fixes #471, closes #473
References jf/rbenv-gemset#56

Closes #469

[ci skip]

Add ruby-2.1.0-rc1.