Enable goproxy in travis (#1744)
Enable goproxy in travis (#1744)

Trial using the Google module proxy in travis builds.

* in preparation for Golang module transparency
* Make dep fetching faster and more reliable.
Martin2112/trillian,AlCutter/trillian,Martin2112/trillian,gdbelvin/trillian,gdbelvin/trillian,RJPercival/trillian,Martin2112/trillian,gdbelvin/trillian,AlCutter/trillian,RJPercival/trillian,google/trillian,google/trillian