fix(runner): don't mark the job as finished if it's being restarted

This means that the job can be marked as finished immediately after it
was restarted, which can cause multiple notifications or errors when the
build is checking if all jobs are finished (when some are actually
marked as created).

Fix travis-ci/travis-ci#1858.
Merge pull request #88 from travis-ci/hh-dont-finish-restarted-jobs

fix(runner): don't mark the job as finished if it's being restarted