Add `bin/deploy` script

* Help avoid forgetting to run `rake db:migrate`.
* Help avoid forgetting to run `restart` after migrate.
* Allow each app to edit these deploy instructions to fit their
  environments' needs (such as `run rake purge` to clear a cache)
  but maintain the same `./bin/deploy` interface and convention
  that is consistent with our many Middleman and Rails apps.
* Use the shebang line for POSIX sh.
* Default the argument to 'staging'