Time execution output should not have .stdout class

When the 'Output the time it took to execute the script' option is enabled, the `[Finished in (...)s]` message is appended to the actual script output. Currently this final `pre` line also has the `.stdout` class set. I would suggest this is omitted, as the line does not contain actual stdout contents.

A direct advantage is this allows themes and other packages to differentiate between actual stdout contents and what was added by atom-script, and for instance provide a clear visual indication that the script has finished running.

An alternative solution would be to add a `execution-time` class explicitly to the line to make it stand out, but it this seems redundant and the message would still be incorrectly labeled as something that came from stdout.