Remove spin from bundled_commands in 'bundler' plugin.

'spin' should be put into Gemfile. It is not the way spin was designed. From README:

"Spin is designed so that your app never has to know about it. You can use Spin to run your tests while the rest of your team doesn't even know that Spin exists."

https://github.com/jstorimer/spin#how-is-it-different-from-spork