OS X: Fixes check for OpenSSL version

We only care about Apple's OpenSSL when checking the version.  Any
other OpenSSL that happens to be in the PATH doesn't matter.
Update libyaml to 0.1.6

Security fix for CVE-2014-2525

Advisory: http://www.ocert.org/advisories/ocert-2014-003.html
link the latest ruby-build.$SEED.log and ruby-build.$SEED/ to ruby-build.latest.log and ruby-build.latest/ respectively
Merge pull request #534 from josacar/security-CVE-2014-2525

Update libyaml to 0.1.6
Add `rbenv install --skip-existing <VERSION>` option

The `-f` option allows for forcing installation of versions that already
exist in the environment. If that option isn't used, then an interactive
prompt is created which causes problems for non-interactive execution of
the command.

This change adds the `-s/--skip-existing` option to not install versions
that already exist, while still exiting as success.

Closes #543
2.0.0-p451 should use libyaml-0.1.6 too
Merge pull request #544 from bryanstearns/libyaml-0.1.6-for-ruby-2.0.0p451-too

2.0.0-p451 should use libyaml-0.1.6 too
added yaml-0.1.6 to 2.1.1 for CVE-2014-2525
changelog
style
Add test for Rubinius gem binstubs fix

References #433
Avoid sed with extended regexp

Seems like sed 4.1.5 on RHEL 5.3 doesn't support `-E`, and sed on OS X doesn't
support `-r` to activate the extended regexp mode. Best to avoid extended regexp
altogether for compatibility.

Fixes #495
Merge branch 'avoid-sed-regexp'
Update changelog
ruby-build 20140405
Stop maintaining the changelog

All releases information has been migrated to GitHub Releases:
https://github.com/sstephenson/ruby-build/releases

This frees core contributors from having to maintain release notes in version
control, and regular contributors from wondering whether they need to edit
CHANGELOG while submitting a pull request.

Benefits of release notes being outside of version control are that they can be
edited post-release.
Merge pull request #545 from sstephenson/nuke-changelog

Stop maintaining the changelog
link to releases in readme.
Merge pull request #546 from rajiv/patch-1

Link to releases in the Readme
OpenSSL library updated to 1.0.1g

Previous versions of the library contain a vulnerability CVE-2014-0160.