Use bundler to manage vendored dependencies.

The rake init task was quite brittle, namely it didn't work
for me at all. Furthermore, it duplicates an awful lot of Bundler's
functionality. However we most likely *don't* want Bundler to become a
runtime dependency and we still want access to the gem sources for
packaging purposes. For that reason the new init rake task uses bundler
in standalone (no bundler needed at runtime) and installs to the vendor
path.

This patch may also improve startup time by reducing the number of calls
to File.expand_path from 6 down to 3.
Use JRuby to bundle in order to remove hard-vendored json and ffi.

By using JRuby to run the standalone bundle install, we can remove the
hard-vendored json-java and ffi-java gems from the repo. To me this
makes a lot of sense however for the time being, it relies on having
JRuby installed and running via RVM (or similar) in development in
addition to the Redcar vendored jruby-complete jar. I'm bugging people
about using MRI to install JRuby deps but it doesn't look hopeful.

If everybody hates needing two JRubies, maybe someone can rewrite the
command to use the vendored one, but that still requires getting bundler
in advance. :(
Allow running `rake init` from MRI, load custom ruby-git
Use Bundler to manage vendored dependencies