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

https://bugzilla.mozilla.org/show_bug.cgi?id=1057449
I don't understand what this does. If we want to silent the output, can't we just use >> /dev/null ? 

And if we want to keep track of what's going on, we could just : 

```
echo 'installing gevent'; $(INSTALL) gevent >> /dev/null
```