Enable ipv6 for testing

.travis.yml

@@ -9,6 +9,11 @@ install:
  - pip list
 before_script:
   - openssl version -a
+  # Add an IPv6 config - see the corresponding Travis issue
+  # https://github.com/travis-ci/travis-ci/issues/8361
+  - if [ "${TRAVIS_OS_NAME}" == "linux" ]; then
+      sudo sh -c 'echo 0 > /proc/sys/net/ipv6/conf/all/disable_ipv6';
+    fi
 script:
  - python -m pytest -x plugins/CryptMessage/Test
  - python -m pytest -x plugins/Bigfile/Test