Fix error when deploying to google app engine

github.com/jinzhu/gorm/postgres.go:103:21: composite struct literal database/sql.NullString with unkeyed fields

postgres.go

@@ -100,7 +100,7 @@ func (h Hstore) Value() (driver.Value, error) {
 	}
 
 	for key, value := range h {
-		hstore.Map[key] = sql.NullString{*value, true}
+		hstore.Map[key] = sql.NullString{String: *value, Valid: true}
 	}
 	return hstore.Value()
 }