This fix fixes version fetched from `go get` so
that versions are guarded.

github.com/mholt/caddy              v0.10.10
github.com/miekg/dns                v1.0.3
github.com/prometheus/client_golang v0.8.0
golang.org/x/net                    release-branch.go1.9 (branch)
golang.org/x/text                   e19ae1496984b1c655b8044a65c0300a3c878dd3

This fix fixes 1368.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>