Fix for 'uninitialized constant YAML' on ruby-head

* We need to require 'yaml' ourselves. Bundler version 1.3.0, the only
  version of Bundler compatible with Ruby 2.0.0 HEAD, loads Psych
  lazily.

  We should no longer rely on YAML being defined in a Bundler
  environment.

Signed-off-by: Alex Coles <alex@alexbcoles.com>