build: Use the official Go module proxy for Travis-CI runs (#22711)

This gets us slightly faster test runtimes and also insulates us better
from downtime of individual upstreams, in favor of being dependent only
on one specific upstream (the Go module proxy itself, currently run by
Google).

.travis.yml

@@ -11,7 +11,7 @@ go:
 # add TF_ETCDV3_TEST=1 to run etcdv3 tests
 # if added, TF_ETCDV3_ENDPOINTS must be set to a comma-separated list of (insecure) etcd endpoints against which to test
 env:
-  - CONSUL_VERSION=0.7.5 GOMAXPROCS=4 GO111MODULE=on
+  - CONSUL_VERSION=0.7.5 GOMAXPROCS=4 GO111MODULE=on GOPROXY=https://proxy.golang.org/
 
 # Fetch consul for the backend and provider tests
 before_install: