Add updated field on Action struct (#129)

* Add updated field on Action struct
* Set omitempty on `updated` field
diff --git a/whisk/action.go b/whisk/action.go
index d8fa2ef..9935871 100644
--- a/whisk/action.go
+++ b/whisk/action.go
@@ -41,6 +41,7 @@
 	Error       string      `json:"error,omitempty"`
 	Code        int         `json:"code,omitempty"`
 	Publish     *bool       `json:"publish,omitempty"`
+	Updated     int64       `json:"updated,omitempty"`
 }
 
 type Exec struct {