Remove ruby 2.0 deprecated StringIO#lines by using #each_line.

StringIO#each_line without a block returning an Enumerator exists on ruby 1.9.3 so we can start enjoying this now and remove the 2.0 deprecation.

See: http://www.ruby-doc.org/stdlib-1.9.3/libdoc/stringio/rdoc/StringIO.html#method-i-each_line

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
```