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/thejerf/suture/.travis.yml

@@ -0,0 +1,9 @@
+language: go
+go:
+  - 1.1
+  - 1.2
+  - 1.3
+  - 1.4
+  - 1.5
+  - 1.6
+  - tip