`assert_success` doesn't accept an argument via stdin

add jruby-1.7.15

add jruby 1.7.14 build

Read ruby-build revision from git if available

Avoid cloning the whole project's history which we'll never need

But don't assume that RUBY_BUILD_ROOT is where ruby-build's own files
reside, since RUBY_BUILD_ROOT can be overriden with alternate definition
files location.

Auto-discovery of build definitions from rbenv plugins

add ruby-1.9.2-p330

The `share/ruby-build/` directory from each rbenv plugin, if it exists,
is added to RUBY_BUILD_DEFINITIONS automatically during `rbenv install`.

Support extra definitions paths via RUBY_BUILD_DEFINITIONS

Closes #609

This is a colon-separated list of directories that will get searched for
build definition in order from left to right.

This is to allow specifying an alternate path where `share/ruby-build/*`
definitions reside. Useful in tests as well.

Add link to suggested build environment to README

Add rbx-2.2.10

add jruby-1.7.13

Add rbx-2.2.9, 2.2.8 was skipped due to broken wrappers.

Fix `fetch_git` with `--keep`

Update existing git repo if exists, or clone if not exists.
The destination repo might exist if the `--keep` was supplied for `ruby-build`.

OpenSSL library updated to 1.0.1h

Previous versions of the library contain a vulnerability CVE-2014-0224.

The script handles bumping up the version number, committing, tagging
and updating the Homebrew formula. The last step requires `hub` to be
installed in order to submit a pull request to Homebrew.

/cc @sferik @hsbt

[ci skip]

Fixes #548, closes #551