Allow dragging of tab when there is only one tab.

The tabs should be able to be draggale out of the application at any
time, and if there is only one tab it should not be able to be dropped
in the same window.

src/packages/tabs/lib/tab-bar-view.coffee

@@ -77,11 +77,8 @@ class TabBarView extends View
     (@paneContainer.getPanes().length > 1) or (@pane.getItems().length > 1)
 
   onDragStart: (event) =>
-    unless @shouldAllowDrag(event)
-      event.preventDefault()
-      return
-
-    event.originalEvent.dataTransfer.setData 'atom-event', 'true'
+    if @shouldAllowDrag()
+      event.originalEvent.dataTransfer.setData 'atom-event', 'true'
 
     el = $(event.target).closest('.sortable')
     el.addClass 'is-dragging'