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
Change protocol of the ruby-lang.org server from HTTP to FTP
Merge pull request #388 from markprzepiora/master

Change server protocol of the ruby-lang.org server in the MRI definitions from HTTP to FTP
ruby-build 20130806
Homebrew homepage location changed
Merge pull request #391 from yyuu/homebrew-homepage-location-changed

Homebrew homepage location changed
Don't set the Rubinius gems directory to the prefix

This causes tricky issues with Bundler. Bundler assumes that the
configure gem path *only* contains gems. When using this configuration
option, this is not the case.

What happens is that Bundler then removes the path for the Ruby stdlib
from the load path, because it is located inside what is configured as
the gems directory. This then breaks for example Rails applications when
using ruby-build to build Rubinius.

Also see: https://github.com/rubinius/rubinius/issues/2544
Merge pull request #398 from rubinius/fix_rubinius_gems_dir

Don't set the Rubinius gems directory to the prefix
use www.dnsbalance.ring.gr.jp
Merge pull request #399 from hsbt/use-ring-mirror

[WIP] use www.dnsbalance.ring.gr.jp
Reformat LICENSE and REAMDE

Wrap at 80 characters.
ruby-build 20130901
Add description of `RUBY_CFLAGS` to README.
manually reverted using mirror site. and use https protocols
use http, because http://ftp.ruby-lang.org is deliveried by fastly
rename hostname, ftp.ruby-lang.org is used by ftp protocol too.
Merge pull request #409 from hsbt/revert-using-mirror

Manually reverted using mirror site. and use http protocols
Merge pull request #408 from alloy/patch-1

Add description of `RUBY_CFLAGS` to README.
ruby-build 20130907
Fix typo in the README file