Bug 1057449 — Hide loadtest build travis logs except for errors.

https://bugzilla.mozilla.org/show_bug.cgi?id=1057449
We want to display the logs only in case of error.
This will : 
  - install everything and store the logs in the `loadtest.log` file,
  - drop the file if everything when correctly or
  - display the content of the file in case an error occured at some point.