NOT FOR MERGE: Log a message if we don't receive any output from `instru...

...ments`

in a few minutes. (refs #147)

This is not for merge but rather just for debugging #147.
(I'm just submitting a pull request to get Travis to run the tests.)
It might be a good idea to add some sort of timeout to `subliminal-instrument`,
at the expiration of which it would either log a message like in this commit
or even abort, but that would take some more thought and also would need
to be configurable by the user.