Fixes #667, closes #668

When requested via `-h|--help`, usage text should be displayed on stdout
and the exit status should be 0.

When usage text is shown due to invalid arguments, it should be
printed to stderr and exit status should be 1.

Use exactly the same arguments that would have `ruby-build` succeed at
executing, but add one more extra to test argument counting.

- rbenv-install accepts 0 or 1 argument
- rbenv-uninstall accepts 1 argument
- ruby-build accepts 2 arguments

JRuby 9000 is now 9.0.0.0 (official version number for release).

`assert_failure` only accepts an expected message as argument, and only
if it matches the output in full.

`script/test` will now only attempt to call `script/mirror` if either
TRAVIS_PULL_REQUEST or TRAVIS_COMMIT_RANGE is set.

Avoids failures caused by somebody's environment possibly having
RUBY_CONFIGURE_OPTS exported to have a value.

Mention 'subversion' in error message when svn is missing

The current error message for missing subversion is "error: please install \`svn\` and try again", leading the user to try to find and install a package named "svn" (eg. `aptitude install svn for # debian / ubuntu). Changing it to "subversion" can help users unaware that svn is a binary installed by the subversion package, get through the error.

Add jruby-1.7.16.1

1.7.16.1 is a security fix release for CVE-2014-8080. All users are strongly recommended to upgrade.

http://jruby.org/2014/10/28/jruby-1-7-16-1.html

Setting CC makes sure that we never have to perform the Darwin-specific
check for whether we should default to `CC=clang` and thus break `uname`
stubs in unrelated tests.

CI never caught this because Travis already has CC set to gcc-4.6.

[ci skip]

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'