Travis CI: Yet another attempt to get IPv6 loopback running

Try fix from comment
https://github.com/travis-ci/travis-ci/issues/8361#issuecomment-350090030

.travis.yml

@@ -7,8 +7,6 @@
 # The "precise=true"+"sudo=required" environment seems to lack IPv6 support.
 # According to some tests, all "sudo=required" environments do not support IPv6, see 
 # https://github.com/travis-ci/travis-ci/issues/8361#issuecomment-328263113
-# The container environments for "sudo=false" support IPv6 localhost [::1] 
-# connections for server/client test. Thus, all tests with ENABLE_IPV6=YES
 #
 
 
@@ -66,6 +64,7 @@ before_script:
   # https://github.com/travis-ci/travis-ci/issues/8361#issuecomment-345074513
   - if [[ "${ENABLE_IPV6}" == "YES" ]]; then 
       echo "IPv6 on Travis may fail";
+      sudo sh -c 'echo 0 > /proc/sys/net/ipv6/conf/all/disable_ipv6';
     fi
   - pwd
   - ls -la
@@ -100,7 +99,7 @@ before_script:
     -DCIVETWEB_C_STANDARD=${C_STANDARD}
     -DCIVETWEB_CXX_STANDARD=${CXX_STANDARD}
     -DCIVETWEB_ALLOW_WARNINGS=${ALLOW_WARNINGS}
-    -DCIVETWEB_ENABLE_IPV6=NO
+    -DCIVETWEB_ENABLE_IPV6=${ENABLE_IPV6}
     ${ADDITIONAL_CMAKE_ARGS}
     ..
   - ls -la