cubicweb-varnish #3603303 update to cw 3.15 API [validation pending]

use cubicweb.predicates instead of cubicweb.selectors

prioritynormal
typetask
done in0.2.1
load0.100
load left0.000
closed by#54d45122a85f import predicates instead of deprecated selectors module (closes #3603303)