source tarball has invalid link

There's a symlink in the repo:
traffic_stats/vendor/github.com/apache/incubator-trafficcontrol/traffic_ops/client -> ../../../../../../traffic_ops/client

When the tarball is created, it uses tar's `--transform` option which breaks that symlink. Fix is trivial, but needs to be backported to 2.0.x because it will break anyone trying to build traffic_stats from the tarball.