Clean up asset precompilation output from travis logs
Clean up asset precompilation output from travis logs

Asset precompilation generates a lot of logs, making it slower to
scroll down to see the commonly relevant information, knowing what spec
has failed

Redirecting standard error and standard output to dev/null helps reach
this information faster

CDJ11/CDJ,CDJ11/CDJ,CDJ11/CDJ,CDJ11/CDJ