go.mod, go.sum: add back in after v2 module renaming

Change-Id: I4366596203940ab2d1fb20f88bc40e95a57157c4
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/177157
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

go.mod

@@ -0,0 +1,5 @@
+module github.com/golang/protobuf
+
+go 1.9
+
+require google.golang.org/protobuf v0.0.0-20190514172829-e89e6244e0e8

go.sum

@@ -0,0 +1,2 @@
+google.golang.org/protobuf v0.0.0-20190514172829-e89e6244e0e8 h1:W21IHLlUZTOSWPAugb7YF/zA4lu4QlPm7T5JwlKYstQ=
+google.golang.org/protobuf v0.0.0-20190514172829-e89e6244e0e8/go.mod h1:791zQGC15vDqjpmPRn1uGPu5oHy/Jzw/Q1n5JsgIIcY=