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

consul/consul,AyuntamientoPuertoReal/decidePuertoReal,AjuntamentdeCastello/consul,AyuntamientoMadrid/participacion,AyuntamientoMadrid/participacion,consul/consul,usabi/consul_san_borondon,lalibertad/consul,CDJ11/CDJ,deivid-rodriguez/participacion,deivid-rodriguez/participacion,consul/consul,AjuntamentdeCastello/consul,AyuntamientoMadrid/consul,consul/consul,usabi/consul_san_borondon,consul/consul,AyuntamientoMadrid/consul,CDJ11/CDJ,AyuntamientoMadrid/consul,lalibertad/consul,CDJ11/CDJ,AyuntamientoMadrid/participacion,usabi/consul_san_borondon,AjuntamentdeCastello/consul,deivid-rodriguez/participacion,AjuntamentdeCastello/consul,usabi/consul_san_borondon,lalibertad/consul,AyuntamientoPuertoReal/decidePuertoReal,lalibertad/consul,deivid-rodriguez/participacion,AyuntamientoPuertoReal/decidePuertoReal,CDJ11/CDJ,AyuntamientoMadrid/consul,AyuntamientoMadrid/participacion