Fails to notice output or completion after long silence on Travis + Linode

If your task does not produce output for a long time (10 minutes, in my case), spread fails to notice the task is working again, and its completion.

See [a build](https://travis-ci.org/Saviq/mir/builds/293169770) where the last few entries say "Output unchanged", and the job times out. It [passes fine](https://travis-ci.org/Saviq/mir/builds/293281230) when [wrapped](https://github.com/Saviq/mir/commit/1415c7c029c223ce0ac9893aeea02ab6d7c47d1a) in a 1-minute interval echo.