Call clearInterval for the currentTimer if one exists

Prevents a double-click from causing the logo to sweep forever after a
load completes.

app/assets/javascripts/logo.js.coffee

@@ -21,6 +21,7 @@ start = ->
   clearHighlights()
   pieceIndex = 0
   pieces.reverse() unless pieces[0] == firstPiece
+  clearInterval(currentTimer) if currentTimer
   currentTimer = setInterval(work, delay)
 
 stop = ->