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/prometheus/procfs/.travis.yml

@@ -0,0 +1,9 @@
+sudo: false
+
+language: go
+
+go:
+- 1.9.x
+- 1.x
+
+go_import_path: github.com/prometheus/procfs