Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

core/themes/classy/templates/field/field.html.twig

@@ -56,7 +56,7 @@
   {% if multiple %}
     <div{{ attributes.addClass(classes, 'field__items') }}>
       {% for item in items %}
-        <div{{ item.attributes.addClass(classes, 'field__item') }}>{{ item.content }}</div>
+        <div{{ item.attributes.addClass('field__item') }}>{{ item.content }}</div>
       {% endfor %}
     </div>
   {% else %}