Update whenever gem

This updates the whenever gem from version 0.7.3 to 0.8.4.

Without this update, I'm running into this error:

```
  * 2013-10-21 12:51:04 executing `whenever:update_crontab'
  * executing "cd /home/deploy/releases/20131021105104 && bundle exec whenever --update-crontab practicing-ruby --set environment=development --roles db"
    servers: ["practicingruby.local"]
    [practicingruby.local] executing command
*** [err :: practicingruby.local] /home/deploy/shared/bundle/ruby/2.0.0/gems/whenever-0.7.3/bin/whenever:36:in `<top (required)>'
*** [err :: practicingruby.local] :
*** [err :: practicingruby.local] invalid option: --roles
*** [err :: practicingruby.local] (
*** [err :: practicingruby.local] OptionParser::InvalidOption
*** [err :: practicingruby.local] )
*** [err :: practicingruby.local] from /home/deploy/shared/bundle/ruby/2.0.0/bin/whenever:23:in `load'
*** [err :: practicingruby.local] from /home/deploy/shared/bundle/ruby/2.0.0/bin/whenever:23:in `<main>'
    command finished in 585ms
*** [deploy:update_code] rolling back
```