liquid `for` attribute should be `reversed`, not `reverse`

[osteele]

See the Liquid documentation https://help.shopify.com/themes/liquid/tags/iteration-tags#for (search for reversed), and the Shopify/liquid test:

    assert_template_result('321', '{%for item in array reversed %}{{item}}{%endfor%}', assigns)

To reproduce:

	template, err := liquid.ParseString("{% for item in array reversed %}{{item}}{% endfor%}", nil)
	if err != nil {
		fmt.Println("err:", err)
	} else {
		writer := new(bytes.Buffer)
		template.Render(writer, map[string]interface{}{
			"array": []int{1, 2, 3},
		})
		fmt.Println(writer)
	}

Expected:
321

Actual (once #13 is fixed):
err: "reversed" is an unknown attribute in for tag ("{% for item in array reverse %}" - line 1)