Fix django 1.6 line.has_visible_field field

jet/templates/admin/includes/fieldset.html

@@ -5,7 +5,7 @@
         <div class="description">{{ fieldset.description|safe }}</div>
     {% endif %}
     {% for line in fieldset %}
-        <div class="form-row{% if line.fields|length_is:'1' and line.errors %} errors{% endif %}{% if not line.has_visible_field %} hidden{% endif %}{% for field in line %}{% if field.field.name %} field-{{ field.field.name }}{% endif %}{% endfor %}">
+        <div class="form-row{% if line.fields|length_is:'1' and line.errors %} errors{% endif %}{% if line.has_visible_field == False %} hidden{% endif %}{% for field in line %}{% if field.field.name %} field-{{ field.field.name }}{% endif %}{% endfor %}">
             {% if line.fields|length_is:'1' %}{{ line.errors }}{% endif %}
             {% for field in line %}
                 <div{% if not line.fields|length_is:'1' %} class="field-box{% if field.field.name %} field-{{ field.field.name }}{% endif %}{% if not field.is_readonly and field.errors %} errors{% endif %}{% if field.field.is_hidden %} hidden{% endif %}"{% elif field.is_checkbox %} class="checkbox-row"{% endif %}>