解决“You have already activated rake 0.9.6, but your Gemfile requires
rake 0.9.2.2. Prepending `bundle exec` to your command may solve this.”
的问题：

http://johnmorales.com/blog/2014/01/06/octopress-you-have-already-activated-rake-but-your-gemfile-requires-rake-dot-dot-dot-prepending-bundler-exec-needs-a-command-to-run-to-your-command-may-solve-this/