Update libyaml to version 0.1.5

Closes https://github.com/sstephenson/ruby-build/issues/499.
ruby-build 20140204
Add warning about CVE-2013-6393 to CHANGELOG [ci skip]

Actually resolves https://github.com/sstephenson/ruby-build/issues/499.
Install libyaml 0.1.5 for 2.x rubies also

This should plug the vulnerability to CVE-2013-6393 (and fix #504)
that can still occur in certain systems: If the ruby build process
couldn't find a libyaml that worked, it would build its own vendored
libyaml, which was 0.1.4 (and is vulnerable).

Instead, specify that the build always should install the latest
libyaml & build against that.
Add JRuby 9000 snapshot.
Merge pull request #505 from chrisseaton/jruby-9000-dev

Add JRuby 9000 snapshot.
Added support for rbx-2.2.5.
Merge pull request #506 from anthonator/master

Add support for rbx-2.2.5
Make whitespace consistent across build definitions
ruby-build 20140210
Require Java 7 for jruby-9000-dev.
Merge pull request #507 from chrisseaton/jruby-9000-dev-check-java

Require Java 7 for jruby-9000-dev.
Simplify Java 7 detection and error message
Use `colorize` helper for `require_gcc`

Ensures that color is off if the stderr stream is not a terminal
Update MagLev branch from master-1.9 to master
Merge pull request #504 from antifuchs/add-libyaml-to-2.x-rubies

Leaves rubies >= 2.0 vulnerable to CVE-2013-6393
Merge pull request #508 from johnnyt/maglev

Update MagLev branch from master-1.9 to master
ruby-build 20140214 :cupid:
Add mruby 1.0.0.
Merge pull request #510 from Red54/master

Add mruby 1.0.0.