godeps: update vmware/govmomi to v0.20 release

vendor/github.com/vmware/govmomi/vapi/internal/internal.go

@@ -51,15 +51,13 @@ func (o AssociatedObject) Reference() types.ManagedObjectReference {
 
 // Association for tag-association requests.
 type Association struct {
-	TagID    string            `json:"tag_id,omitempty"`
 	ObjectID *AssociatedObject `json:"object_id,omitempty"`
 }
 
 // NewAssociation returns an Association, converting ref to an AssociatedObject.
-func NewAssociation(tagID string, ref mo.Reference) Association {
+func NewAssociation(ref mo.Reference) Association {
 	obj := AssociatedObject(ref.Reference())
 	return Association{
-		TagID:    tagID,
 		ObjectID: &obj,
 	}
 }