use go 1.13.x to build go-multierror

.travis.yml

@@ -3,10 +3,10 @@ sudo: false
 language: go
 
 go:
-  - 1.x
+  - 1.13.x
 
 branches:
   only:
     - master
 
-script: env GO111MODULE=on make test testrace
+script: make test testrace

go.mod

@@ -1,3 +1,5 @@
 module github.com/hashicorp/go-multierror
 
 require github.com/hashicorp/errwrap v1.0.0
+
+go 1.13

prefix_test.go

@@ -27,6 +27,9 @@ func TestPrefix_NilError(t *testing.T) {
 func TestPrefix_NonError(t *testing.T) {
 	original := errors.New("foo")
 	result := Prefix(original, "bar")
+	if result == nil {
+		t.Fatal("error result was nil")
+	}
 	if result.Error() != "bar foo" {
 		t.Fatalf("bad: %s", result)
 	}