The error seems pretty explanatory:
You have already activated rspec-core 2.13.0, but your Gemfile requires rspec-core 2.11.1. Using bundle exec may solve this. (Gem::LoadError)
Either adjust the version of your rspec-rails (containing rspec-core) or run with bundle exec.
Solution:
Within your gemfile, change:
gem 'rspec-rails', '2.11.0'
To:
gem 'rspec rails', '2.13.0'