Fix `irb` on rbx-2.2.1

Install the missing `rubysl-tracer` gem

References rubinius/rubinius#2780
Add `--patch` flag to apply a Ruby patch from stdin

If `-p|--patch` flag was set while invoking `ruby-build` or
`rbenv install`, ruby-build will use `patch -p0 -i -` to apply a patch
from stdin to Ruby, Rubinius, or JRuby source code before running the
rest of `build_package_*` commands.

References #443
Shorten `rbenv install` usage synopsis

It was getting out of hand, so just use the single-character flags which
are documented below anyway.
Auto-detect and link to Homebrew's readline

I thought this was not necessary, but a number of people had problems
when linking to OS X's "readline" (actually Editline wrapper):

- Some components of Pry wouldn't work
- Writing literal Unicode characters was not possible #379
- The compilation would downright fail in some cases #82 #461

Fixes #461
URL hack to fix recognizing that Topaz archive is bz2

On OS X and some other platforms it doesn't matter, as `tar`
automatically detects the compression method, but on platforms that
don't it's important to declare the archive format. For ordinary
tarballs the extension provides this info, but since Topaz archive
doesn't have an extension in the URL, we add a fake one via a dummy
query string parameter.

Fixes #357
Clarify which `rbenv install` flags get forwarded to `ruby-build`
ruby-build 20131211
Add ruby-2.1.0-rc1.
Merge pull request #474 from havenwood/2.1.0-rc1

Add ruby-2.1.0-rc1.
Add examples how to use the `--patch` option to the Readme

[ci skip]
Merge branch 'apply-patch'

Closes #469
Transform JRuby shebangs to absolute paths, fixing recursion within rbenv

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
ruby-build 20131220
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.