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, now lazy-loads
  Psych. We should no longer rely on YAML being defined in a Bundler
  environment.

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