Fix ruby 2.0 StringIO#lines deprecation warning.

Seen in travis output:
```
vmdb/spec/lib/vmdb/logging/mirrored_logger_spec.rb:38: warning: StringIO#lines is deprecated; use #each_line instead
vmdb/spec/lib/vmdb/logging/mirrored_logger_spec.rb:31: warning: StringIO#lines is deprecated; use #each_line instead
```