AmazonFlexPay usage broke, so I fixed it

At one point, it seems that AmazonFlexPay returned an object on which you could call url. It doesn't do that anymore (https://github.com/kickstarter/amazon_flex_pay/commit/512d077b563c573122a43392e875752ca78b4e86). It returns the url as a String. So, method_not_found errors (calling String#url) were a-flyin'. I updated the call to AmazonFlexPay.multi_use_pipeline, which you can now redirect_to directly, and removed the @pipeline and @pipeline.url business altogether.