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

Update libyaml to 0.1.6

Security fix for CVE-2014-2525

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

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

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

This reverts commit 6cf348e4. See
https://github.com/sstephenson/ruby-build/issues/526#issuecomment-37889552.

Closes https://github.com/sstephenson/ruby-build/issues/526.

Add Rubinius 2.2.6.

Fixes #517

Better to use -f than -e to check for a user-specified definition file.
With -e, if the user accidentally types the name of a directory (or, far
less likely, a device file) with the same name as the Ruby they're
trying to install, they end up with ruby-build doing absolutely nothing
at best, or an error message that could be pretty confusing at worst.

Closes #514

JRuby 9000 (the next major release) includes a backend that supports a
new JVM JIT compiler called Graal. Graal is based on OpenJDK but hasn't
been released yet.

http://openjdk.java.net/projects/graal/

Added a JRuby 1.7.11 recipe.

Fix Java 7 detection

Java on most (all?) platforms directs `-version` output to STDERR.

Add mruby 1.0.0.

Update MagLev branch from master-1.9 to master

Leaves rubies >= 2.0 vulnerable to CVE-2013-6393

Ensures that color is off if the stderr stream is not a terminal

Require Java 7 for jruby-9000-dev.