Update world_pay.html

As per #95 if you're not going to use the world_pay templatetag then the default template needs to be updated to accept the form from the integration object rather than expect it directly in the context.

In the world_pay tag we were have:

        form_str = render_to_string("billing/world_pay.html",
                                    {"form": int_obj.generate_form(),
                                     "integration": int_obj}, context)

In the render_integration tag we pass:

        form_str = render_to_string(
                int_obj.template, {
                    "integration": int_obj
                },context)

Therefore the default worldpay template was not receiving the 'form' but was receiving the 'integration'