Try openssl from Homebrew when calculating SHA-256 checksums

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

Fixes #656
Fix Rubinius build when libyaml is present in Homebrew

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.
Work around Rubinius LLVM incompatibilities on Yosemite

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.
Fix rbx-1.2.4 setup phase

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.
Enable Rubinius builds to re-use cached LLVM dependencies between builds

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.
Avoid endless recursion when rbx is installed over existing one

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.