Update to RSpec 2.0

spec_helper.rb generated with `rspec --init`
With help from:
http://pivotallabs.com/upgrading-your-rakefile-from-rspec-1-3-to-rspec-2/
https://www.relishapp.com/rspec/rspec-rails/v/2-5/docs/upgrade
https://github.com/lazyatom/gem-this/pull/6
Fix spec: backtrace includes method info