go.mod: Update github.com/golang/protobuf to v1.3.1 (#204)

Fixes #194

Author: sbuss

go.mod

@@ -1,7 +1,7 @@
 module google.golang.org/appengine
 
 require (
-	github.com/golang/protobuf v1.2.0
+	github.com/golang/protobuf v1.3.1
 	golang.org/x/crypto v0.0.0-20190605123033-f99c8df09eb5 // indirect
 	golang.org/x/net v0.0.0-20190603091049-60506f45cf65
 	golang.org/x/sys v0.0.0-20190606165138-5da285871e9c // indirect

go.sum

@@ -1,5 +1,7 @@
 github.com/golang/protobuf v1.2.0 h1:P3YflyNX/ehuJFLhxviNdFxQPkGK5cDcApsge1SqnvM=
 github.com/golang/protobuf v1.2.0/go.mod h1:6lQm79b+lXiMfvg/cZm0SGofjICqVBUtrP5yJMmIC1U=
+github.com/golang/protobuf v1.3.1 h1:YF8+flBXS5eO826T4nzqPrxfhQThhXl0YzfuUPu4SBg=
+github.com/golang/protobuf v1.3.1/go.mod h1:6lQm79b+lXiMfvg/cZm0SGofjICqVBUtrP5yJMmIC1U=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
 golang.org/x/crypto v0.0.0-20190605123033-f99c8df09eb5/go.mod h1:yigFU9vqHzYiE8UmvKecakEJjdnWj3jj499lnFckfCI=
 golang.org/x/net v0.0.0-20180724234803-3673e40ba225 h1:kNX+jCowfMYzvlSvJu5pQWEmyWFrBXJ3PBy10xKMXK8=