feed.xml is broken by breaking change of Padrino

[r7kamura.github.io/feed.xml](https://github.com/r7kamura/r7kamura.github.io/blob/f90cb95442d2542739938ce28e7b4618fd1cad17/feed.xml) is broken because Padrino has implicitly applied layout in rendering builder, markdown etc. since version 0.12.2 (this behavior is caused by [72acd33](https://github.com/padrino/padrino-framework/commit/72acd33efd95011281a71c5c1083dfc58ca43144)).

This pull request is workaround for it.