* Update vendor directory for latest changes.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Update Gopkg.toml to remove the constraint on zipkin-go-opentracing

As the issue on zipkin-go-opentracing has been fixed. See #1193
for details.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>