Fix build with latest travis images

We use the garnet image, which got updated as described here:
  https://blog.travis-ci.com/2017-06-21-trusty-updates-2017-Q2-launch

One of the updates removed the fakeroot package from the base, so this
update re-adds it explicitly.

.travis.yml

@@ -23,6 +23,7 @@ addons:
     - libxml2-utils
     - traceroute
     - ipset
+    - fakeroot
 #
 # Setup environment
 before_install: