Use double quotes in CoffeeScript files

As we do in the rest of the application.

Note we cannot add a rule enforcing double quotes because CoffeeScript
Lint does not have such rule.

app/assets/javascripts/checkbox_toggle.js.coffee

@@ -1,10 +1,10 @@
 App.CheckboxToggle =
 
   initialize: ->
-    $('[data-checkbox-toggle]').on 'change', ->
+    $("[data-checkbox-toggle]").on "change", ->
       $this = $(this)
-      $target = $($this.data('checkbox-toggle'))
-      if $this.is(':checked')
+      $target = $($this.data("checkbox-toggle"))
+      if $this.is(":checked")
         $target.show()
       else
         $target.hide()