appveyor: use stack: go 1.13.3

appveyor.yml

@@ -6,7 +6,8 @@ clone_depth: 1
 environment:
   GO111MODULE: on
   GOPROXY: "https://proxy.golang.org,direct"
-  GOVERSION: 1.13.3
+
+stack: go 1.13.3
 
 build: false
 deploy: false