Fixed clearing log
Added tests for clearing log when a job is started or requeued
Merge pull request #214 from Aaron1011/clear_log

Clear log when a job is restarted
Remove unused observer

log property in Travis.Job does not change - we create Log instance
when it's accessed and don't refresh it on any occasion, that's why the
observer on log is not needed.
Always subscribe to log updates

When the job is restart, we will not get any updates unless we're
subscribed to job updates - that's why we need to subscribe even if the
job is already finished.

The other option to fix this would be to subscribe and unsubscribe also
based on the status, but since subscribing to finished jobs does not
cost us anything, I prefer the simpler solution.