Travis: enable apt-get update (that's opt-in now)

Travis runs on a Debian environment, and we need to install packages
such as QEMU in order to run the CI scripts. As of today, we need to
explicitly enable that we want to do "apt-get update" or else it
can't find the packages we need.

.travis.yml

@@ -3,6 +3,7 @@ python: "3.6"
 sudo: required
 addons:
   apt:
+    update: true
     sources:
     - debian-sid
 install: