Travis: Check IPv6 settings

To find out if there is a way to enable IPv6 in the Travis CI environment,
check if IPv6 is disabled there.
See also
https://github.com/travis-ci/travis-ci/issues/8361
https://github.com/travis-ci/travis-ci/issues/8711

.travis.yml

@@ -56,6 +56,7 @@ before_script:
   # Check some settings of the build server (operating system, IPv6 availability, directory)
   - uname -a
   - ifconfig
+  - cat /etc/sysctl.conf
   - pwd
   - ls -la
   - if [ "${TRAVIS_OS_NAME}" == "linux" ]; then