build: Upgraded nodejs to 8 on appveyor

appveyor.yml

@@ -5,7 +5,7 @@ os: Windows Server 2012 R2
 clone_folder: c:\gopath\src\github.com\grafana\grafana
 
 environment:
-  nodejs_version: "6"
+  nodejs_version: "8"
   GOPATH: C:\gopath
   GOVERSION: 1.11