require 'yaml' in object specs

Not all versions of ruby/rspec/rake will load yaml by default, and we don't
explictly require it in the build object specs, which cause the specs to fail
on some hosts. This commit addresses this by requiring yaml before beginning.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Merge pull request #116 from MosesMendoza/require_yaml_in_spec

require 'yaml' in object specs