dns resolution issue on osx worker

[Ilya Shipitsin]

https://travis-ci.org/OpenVPN/openvpn/jobs/169565110#L24

why ?

[Brandon Burton]

DynDNS is being DDOSed and it's impacting large percentages of the Internet's ability to function, https://twitter.com/nytimesbusiness/status/789462007900758017

Also, this is not the appropriate repository to report Travis CI platform
issues, please file such issues in https://github.com/travis-ci/travis-ci/issues in the future

Thanks