Only support installing Rubinius >= 2.0.0 (issue #79)

* Include rubygems as a dependency.
* Removed rake as a dependency, let bundler install the desired version
  of rake.
* Install llvm-dev on Debian/Ubuntu systems.
* Download from http://releases.rubini.us/rubinius-$RUBY_VERSION.tar.bz2.
* Install bundler from rubygems, to ensure we always have the latest
  version of bundler.
* Run bundle install before running ./configure.
Also install llvm on Arch.
Install ruby1.9.1-dev in case we are on an older version of Ubuntu.
Avoid install the rubygems package, as it pulls in ruby1.8 (issue #79).
Experiment with installing gems into vendor/gems (issue #79).
Invoke rake via `bundle exec` to avoid version mismatches (issue #79).
Also run ./configure under `bundle exec` (issue #79).
Indentation
Try to keep code within 80 columns.
Do not install bundler, if it's already present (issue #79).
If the gemdir is writable, do not use sudo (issue #79).

* Always install the most recent version of bundler, so that we satisfy
  rubinius-2.0.0's Gemfile.
Merge branch 'rubinius-2.0.0'