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/pkg/errors/.travis.yml

@@ -0,0 +1,13 @@
+language: go
+go_import_path: github.com/pkg/errors
+go:
+  - 1.4.x
+  - 1.5.x
+  - 1.6.x
+  - 1.7.x
+  - 1.8.x
+  - 1.9.x
+  - tip
+
+script:
+  - go test -v ./...