all: Become a Go module (fixes #5148) (#5384)

* go mod init; rm -rf vendor

* tweak proto files and generation

* go mod vendor

* clean up build.go

* protobuf literals in tests

* downgrade gogo/protobuf

vendor/github.com/AudriusButkevicius/cli/.travis.yml

@@ -0,0 +1,6 @@
+language: go
+go: 1.1
+
+script:
+- go vet ./...
+- go test -v ./...