revert command we can use

The deploy:rollback works by copying files around which isn't working because we didn't originally install it with capistrano so those rollback files aren't there. This adds a deploy:revert that calls git revert to undo the code change effect of deploy:update (which is implemented with git pull).

I'm tempted to try reinstalling canvas with capistrano from the beginning, but I worry that will destroy our production setup. Maybe it'd be worth trying after this semester though, when users aren't actively on it. Until then, I think the git pull and revert workflow is a good solution for our code changes - it's the same thing we'd do on local development to undo an unwanted change.