Handle process sorting when one-off's present

To prevent this error:
```
    Error:       undefined method `captures' for nil:NilClass (NoMethodError)
    Backtrace:   /Users/nickmerwin/.heroku/plugins/heroku-dyno-size/lib/heroku/command/ps-size.rb:46:in `block (2 levels) in index'
```

Under these circumstances:
```
=== run: one-off processes
run.2575 (1X): up 2013/04/02 11:47:25 (~ 40m ago): `rails c`
run.8308 (1X): up 2013/04/02 11:53:59 (~ 34m ago): `rails c`

=== web (1X): `bundle exec unicorn -p $PORT -c ./config/unicorn.rb`
web.1: up 2013/04/01 16:19:38 (~ 20h ago)
web.2: up 2013/04/01 16:19:53 (~ 20h ago)
web.3: up 2013/04/01 16:19:44 (~ 20h ago)

=== worker (2X): `bundle exec rake jobs:work`
worker.1: up 2013/04/02 11:51:47 (~ 36m ago)
worker.2: up 2013/04/02 11:51:52 (~ 36m ago)
worker.3: up 2013/04/02 12:27:58 (~ 9s ago)
worker.4: up 2013/04/02 12:28:00 (~ 7s ago)
```