Use Homebrew libyaml if it is available

This checks for Homebrew's yaml to see if it
is available and that `--with-libyaml-dir` hasn't
been specified.

Related issue: #379
Add Rubinius 2.0.0
Download from official Rubinius releases site
run bundle before configure rubinius
Merge pull request #418 from jalkoby/master

Run bundle before configure rubinius
Add JRuby 1.7.5
Merge pull request #421 from deepj/master

Add JRuby 1.7.5
ruby-build 20131008
Add rbx-2.1.0
Merge pull request #430 from txus/master

Add Rubinius 2.1.0
Add rbx-2.1.1
Fix Rubinius gem binstubs path

Rubinius 2 insists that it installs RubyGems binstubs into
`PREFIX/gems/bin` instead of `PREFIX/bin`.

This creates complexity for rbenv rehash and exec processes, so we
symlink `gems/bin` into `bin` and have RubyGems create binstubs at a
location that is consistent with other Ruby implementations.

See sstephenson/rbenv#178, sstephenson/rbenv#461
Merge pull request #432 from ProGNOMmers/rbx-2.1.1

Add rbx-2.1.1
Added a definition for JRuby 1.7.6.
Merge pull request #435 from nirvdrum/jruby-1.7.6

Added a definition for JRuby 1.7.6.
Fix Rubinius 2.1.x on Linux by supporting bz2 archives

`tar` on OS X auto-detects the type of compression used, so it doesn't
care whether `-z` or `-j` flags were passed to it for extraction.
However, since latest Rubinius archives are compressed with bzip2, we
must handle this distinction explicitly for the sake of other platforms.

fixes #470
Merge pull request #433 from sstephenson/rbx-fix

Fix Rubinius gem binstubs path
Fix broken test suite

Fixes breakage caused by curl arguments change in
d0912e4fb550490423af42954b715be2f15370ec
configure Travis CI
ruby-build 20131024