Enforce that plot-to-board sets chart as cover

`trollolo burndown --plot-to-board` only added the new chart to
the card. Trello does not always set the new attachment as the
cover. This ensures that the attachment is added as a cover.

Fix #114.

CHANGELOG.md

@@ -4,6 +4,8 @@
 
 * Fix `plot-to-board` option in the burndown command when it is used together
   with `-o`.
+* `burndown --plot-to-board` always sets the burndown chart as the cover.
+  Fix #114.
 
 ## Version 0.0.14
 

lib/burndown_chart.rb

@@ -215,7 +215,9 @@ def update(options)
       board = trello.board(board_id)
       name = options['output'] ? options['output'] : '.'
       name += "/burndown-#{sprint.to_s.rjust(2, '0')}.png"
-      trello.add_attachment(board.burndown_card_id, name)
+      card_id = board.burndown_card_id
+      trello.add_attachment(card_id, name)
+      trello.make_cover(card_id, name)
     end
   end