Upgrade RSpec to 2.13.x

Upgrades RSpec suite to 2.13.x in the gemspec, and fixes a few issues I found after upgrading.

https://github.com/opscode/chef/commit/9530193cd23c57604ad835a06c854e2c032b9904 isn't strictly necessary, but I did need it in order to repro the ARGV-related issues and it seems pretty innocuous.

BTW, I'm doing this because I have no idea what's causing test failures with "undefined method `now` for Object:class", and I'm hoping it was a bug in rspec-mocks that got fixed.