Fix re-downloading tarball when destination file already exists

Previously, curl and wget were instructed to try to resume the download
if the destination file already exists. This is supposed to be done via
the "Range" HTTP header, but doesn't work well with CloudFront:

    HTTP server doesn't seem to support byte ranges. Cannot resume.

CloudFront is supposed to support ranges, so I don't know what's going
on here. It might be failing only in case the existing file is a fully
downloaded tarball?

In any case, this disables resuming downloads and resorts to simply
re-downloading the tarball always, overwriting the existing file.

Fixes #487
Avoid re-downloading if there's a valid tarball in build location

If there already exists a valid tarball in the build location, e.g. as
artefact of a previous install using `--keep`, don't re-download the
file, as there is no need.

References #487
Merge branch 'no-resume-download'
Update libyaml to 0.1.6

Security fix for CVE-2014-2525

Advisory: http://www.ocert.org/advisories/ocert-2014-003.html
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