Travis: Use Blue Box VMs for IPv6 loopback support

Author: luke-jr

.travis.yml

@@ -3,6 +3,12 @@
 #   compiler key (which we don't use anyway). This is worked around for now by
 #   replacing the "compilers" with a build name prefixed by the no-op ":"
 #   command. See: https://github.com/travis-ci/travis-ci/issues/4393
+# - sudo/dist/group are set so as to get Blue Box VMs, necessary for [loopback]
+#   IPv6 support
+
+sudo: required
+dist: precise
+group: legacy
 
 os: linux
 language: cpp