Mixlib's shellout expects streams to have a shift operator

In metal we use the EventsOutputStream and pass that to shellout. Shellout does the following:

```ruby
@live_stdout << chunk if @live_stdout
```
here: https://github.com/opscode/mixlib-shellout/blob/master/lib/mixlib/shellout/unix.rb#L256 - where @live_stdout is an EventsOutputStream object, which then bombs because the object doesn't have a shift operator.

cc @jkeiser