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/rcrowley/go-metrics/.travis.yml

@@ -0,0 +1,18 @@
+language: go
+
+go:
+    - 1.2
+    - 1.3
+    - 1.4
+    - 1.5
+    - 1.6
+    - 1.7
+    - 1.8
+    - 1.9
+
+script:
+    - ./validate.sh
+
+# this should give us faster builds according to 
+# http://docs.travis-ci.com/user/migrating-from-legacy/
+sudo: false