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

Change-Id: Iba5d25391870b4a6014fd90fc79ab101c75ea3c7
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/177042
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..25d8ea0
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,8 @@
+module google.golang.org/protobuf
+
+go 1.9
+
+require (
+	github.com/golang/protobuf v1.2.1-0.20190514181236-7800af189d76
+	github.com/google/go-cmp v0.3.0
+)