Fix ipv6 bind on travis

see https://github.com/travis-ci/travis-ci/issues/8361#issuecomment-350497804

.travis.yml

@@ -1,4 +1,5 @@
 language: php
+sudo: true
 
 php:
   - 7.0
@@ -10,6 +11,9 @@ env:
   global:
     - REPORT_EXIT_STATUS=1
 
+before_script:
+  - sudo sh -c 'echo 0 > /proc/sys/net/ipv6/conf/all/disable_ipv6'
+
 before_install:
   - chmod +x travis-install.sh