we require go 1.13

Make sure we specify go v1.13, not 1.14
diff --git a/go.mod b/go.mod
index 0afe8e6..141cc4c 100644
--- a/go.mod
+++ b/go.mod
@@ -1,5 +1,5 @@
 module github.com/hashicorp/go-multierror
 
-go 1.14
+go 1.13
 
 require github.com/hashicorp/errwrap v1.0.0