fix: add go.mod (#293)

* add go.mod

* The minimum Go requirement is go 1.11

Author: shogo82148

go.mod

@@ -0,0 +1,21 @@
+module firebase.google.com/go
+
+go 1.11
+
+require (
+	cloud.google.com/go v0.47.0 // indirect
+	cloud.google.com/go/firestore v1.0.0
+	cloud.google.com/go/storage v1.1.2
+	github.com/golang/groupcache v0.0.0-20191027212112-611e8accdfc9 // indirect
+	github.com/jstemmer/go-junit-report v0.9.1 // indirect
+	go.opencensus.io v0.22.1 // indirect
+	golang.org/x/exp v0.0.0-20191030013958-a1ab85dbe136 // indirect
+	golang.org/x/net v0.0.0-20191101175033-0deb6923b6d9 // indirect
+	golang.org/x/oauth2 v0.0.0-20190604053449-0f29369cfe45
+	golang.org/x/sys v0.0.0-20191104094858-e8c54fb511f6 // indirect
+	golang.org/x/tools v0.0.0-20191104232314-dc038396d1f0 // indirect
+	google.golang.org/api v0.13.0
+	google.golang.org/appengine v1.6.5
+	google.golang.org/genproto v0.0.0-20191028173616-919d9bdd9fe6 // indirect
+	google.golang.org/grpc v1.24.0 // indirect
+)