Improve govendor testing (#1623)

- Use `govendor list +outside +unused` for finding missing or unused deps and govendor status for catching modified vendor.  
- Remove appengine import

vendor/vendor.json

@@ -1,6 +1,6 @@
 {
 	"comment": "",
-	"ignore": "test",
+	"ignore": "test appengine",
 	"package": [
 		{
 			"checksumSHA1": "spqE5xUEPQp8YV67McMTMAUIilY=",