Closes #646, fixes #641

This fixes `rbenv install -l` displaying each version twice due to
RUBY_BUILD_DEFINITIONS path containing ruby-build's own definitions path
twice: both as an rbenv plugin and by appending its own internal path.

Fixes #651, fixes #648

See https://github.com/rubinius/rubinius/issues/3169

When installing rbx over an existing location, the `gems/bin` directory
will already be a symlink to `bin/` and an attempt to recreate this will
end up in recursion that keeps growing a binstub file until the disk is full.

If RUBY_BUILD_CACHE_PATH is set (typically "`rbenv root`/cache" if it
exists), have Rubinius `./configure` script download prebuilt LLVM
versions into that directory and re-use them if already present.

Rubinius back then didn't have a Gemfile, so don't try to invoke `bundle`.
Instead, rely on the host Ruby to already have rake installed.

Rubinius fails to download a prebuilt LLVM on Yosemite since one is not
available yet. Instead, download the prebuilt version for the previous
OS X release.

This fixes Rubinius 2.2.7+ builds, but the older ones still fail for me
on Yosemite. This could be due to the fact that they're old releases
which are not compatible with never dependencies on the system.

Rubinius never supported the `--with-libyaml-dir` configure option, so
we can never use `--if needs_yaml` since it's incompatible and might
break the build if libyaml was auto-discovered from Homebrew.

Starting from 2.2.0, Rubinius doesn't even need libyaml anymore since it
stopped building psych. Instead it requires users to install psych as a
gem which vendors its own libyaml anyway.

This makes checksums possible on Leopard where `shasum` is unavailable
and system openssl doesn't support `dgst -sha256`.

Fixes #656

Newer MRIs will pick up gcc-4.2 from PATH and use that instead of
`/usr/bin/gcc`. While this worked up till now, it will not work in
Yosemite anymore since Homebrew's apple-gcc42 is generally not
compatible with 10.10.

So when CC has not explicitly been set, set it to `clang` to avoid
searching the PATH for any other gcc versions. This fixes MRI builds on
systems where apple-gcc42 is installed.

This silences the warning that gcc-4.2 produces on Yosemite:

    couldn't understand kern.osversion `14.0.0'

The definitions that use `require_gcc` are not compatible with Apple's
clang-powered `gcc` and need gcc-4.2 from Homebrew. However, builds
using gcc-4.2 fail on Yosemite with a warning:

    couldn't understand kern.osversion `14.0.0'

Although the warning is non-fatal, the build goes to shit from there. It
seems that setting the magical value `MACOSX_DEPLOYMENT_TARGET=10.9`
makes the build work and doesn't seem to have negative consequences.

Use HTTPS URLs to fetch JRuby distfiles.

See https://twitter.com/jruby/status/525726366533443584 which expresses
the desire to see this change.

Fix TMPDIR noexec check to explicitly exit with a zero exit status on success

The TMPDIR check implemented in a4556a73 incorrectly reports that
TMPDIR cannot hold executables on 4.3.11(1) on Ubuntu 14.04.1 LTS.
This is because a script containing no commands returns a non-zero
exit code.  Contrast the following:

  bash 3.2.53(1) on OS X 10.9.5:

    $ bash -c '' && echo SUCCESS
    SUCCESS

  bash 4.3.11(1) on Ubuntu 14.04.1 LTS:

    $ bash -c '' || echo FAIL
    FAIL

This patch modifies the test script to explicitly call `exit 0` to
ensure that a successful exit code is returned if the script executes
successfully.

Upgrade to openssl-1.0.1j

Disable insecure SSL protocols

add jruby-1.7.16

Upgrade OpenSSL library to 1.0.1i

Previous versions of the library contain several vulnerabilities:
https://www.openssl.org/news/secadv_20140806.txt

OS X: Fixes check for OpenSSL version

Fixes #606

Added definition of new rubies

Add 2.2.0-preview1

Show specific upgrade instructions based on environment

Add missing checksums to Rubinius definitions