Avoid double-newlines in salt-call output

When using the salt provisioner with verbose=true, most lines read with an extra newline:
```
[INFO    ] Syncing modules for environment 'base'

[INFO    ] Loading cache from salt://_modules, for base)

```

because the line read has a newline, and emitting the log entry again includes an additional newline.
> Returns a new String with the given record separator removed from the end of str (if present). If $/ has not been changed from the default Ruby record separator, then chomp also removes carriage return characters (that is it will remove \n, \r, and \r\n).

It's the same as `rstrip`, but specifically for \n and \r. rstrip will remove tabs, spaces, and other \s characters