travis: Enable IPv6 on build hosts

Since the move to Google Compute Engine (GCE) IPv6 has been disabled
on the build hosts, causing several tests to fail.  Lets try to get at
least local IPv6 connectivity up again.

.travis.yml

@@ -8,6 +8,9 @@ before_install:
   - ./scripts/test.sh deps
   - ./scripts/test.sh pydeps
 
+before_script:
+  - sudo sysctl -w net.ipv6.conf.all.disable_ipv6=0
+
 script:
   - ./scripts/test.sh