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/oschwald/maxminddb-golang/.travis.yml

@@ -0,0 +1,23 @@
+language: go
+
+go:
+ - 1.4
+ - 1.5
+ - 1.6
+ - 1.7
+ - 1.8
+ - tip
+
+before_install:
+  - "if [[ $TRAVIS_GO_VERSION == 1.7 ]]; then go get -v github.com/golang/lint/golint; fi"
+
+install:
+  - go get -v -t ./...
+
+script:
+  - go test -race -cpu 1,4 -v
+  - go test -race -v -tags appengine
+  - "if [[ $TRAVIS_GO_VERSION == 1.7 ]]; then go vet ./...; fi"
+  - "if [[ $TRAVIS_GO_VERSION == 1.7 ]]; then golint .; fi"
+
+sudo: false