Use own slack fork to fix #937 (#943)

vendor/github.com/nlopes/slack/block_unknown.go

@@ -0,0 +1,14 @@
+package slack
+
+// UnknownBlock represents a block type that is not yet known. This block type exists to prevent Slack from introducing
+// new and unknown block types that break this library.
+type UnknownBlock struct {
+	Type     MessageBlockType `json:"type"`
+	BlockID  string           `json:"block_id,omitempty"`
+	Elements BlockElements    `json:"elements"`
+}
+
+// BlockType returns the type of the block
+func (b UnknownBlock) BlockType() MessageBlockType {
+	return b.Type
+}