travis: no need to disable ipv6 anymore

This was a workaround for an old bug in an old travis base image. Not an
issue anymore

Signed-off-by: Anatole Denis <anatole@unverle.fr>

.travis.yml

@@ -20,11 +20,6 @@ before_install:
   - go get -t -v ./...
   - go get -t -v github.com/stretchr/testify/...
 
-before_script:
-  - if [ "${TRAVIS_OS_NAME}" == "linux" ]; then
-      sudo sh -c 'echo 0 > /proc/sys/net/ipv6/conf/all/disable_ipv6';
-    fi
-
 script: |
   set -x
   case $TEST_SUITE in