Update available dialog: default now 'later', not 'restart' (#1894)

This prevents the user from restarting the app by pressing space on
MacOS,  which happens frequently during the course of normal typing of a
message.

In the future we'll move to a less-obtrusive system a little
more like Chrome, where you choose to do the restart when it is
convenient for you. For now, we minimize the chance of accidental
restart in the middle of typing a message.

app/auto_update.js

@@ -34,8 +34,8 @@ function showUpdateDialog(mainWindow, messages) {
     title: messages.autoUpdateNewVersionTitle.message,
     message: messages.autoUpdateNewVersionMessage.message,
     detail: messages.autoUpdateNewVersionInstructions.message,
-    defaultId: RESTART_BUTTON,
-    cancelId: LATER_BUTTON
+    defaultId: LATER_BUTTON,
+    cancelId: RESTART_BUTTON,
   }
 
   dialog.showMessageBox(mainWindow, options, function(response) {