Update vendor for next release (#1343)

vendor/github.com/slack-go/slack/block_action.go

@@ -6,7 +6,7 @@ package slack
 type ActionBlock struct {
 	Type     MessageBlockType `json:"type"`
 	BlockID  string           `json:"block_id,omitempty"`
-	Elements BlockElements    `json:"elements"`
+	Elements *BlockElements   `json:"elements"`
 }
 
 // BlockType returns the type of the block
@@ -19,7 +19,7 @@ func NewActionBlock(blockID string, elements ...BlockElement) *ActionBlock {
 	return &ActionBlock{
 		Type:    MBTAction,
 		BlockID: blockID,
-		Elements: BlockElements{
+		Elements: &BlockElements{
 			ElementSet: elements,
 		},
 	}