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/chmduquesne/rollinghash/.travis.yml

@@ -0,0 +1,12 @@
+language: go
+go:
+  - "1.10"
+  - "1.9"
+  - "1.8"
+  - "1.7"
+before_install:
+  - go get github.com/mattn/goveralls
+script:
+  - go test -v ./...
+  - go test -bench=. ./...
+  - $HOME/gopath/bin/goveralls -package=./... -service=travis-ci