Updating dependency github.com/google/go-cmp to version v0.3.1

Signed-off-by: Davanum Srinivas <davanum@gmail.com>

vendor/github.com/google/go-cmp/cmp/report_reflect.go

@@ -208,7 +208,6 @@ func (opts formatOptions) FormatValue(v reflect.Value, m visitedPointers) (out t
 func formatMapKey(v reflect.Value) string {
 	var opts formatOptions
 	opts.TypeMode = elideType
-	opts.AvoidStringer = true
 	opts.ShallowPointers = true
 	s := opts.FormatValue(v, visitedPointers{}).String()
 	return strings.TrimSpace(s)