Use `doctype html` instead of `doctype 5`

Hi, in jade v1.0.0 `doctype 5` is deprecated, so I just update the change in `layout.jade`.

http://cl.ly/image/1N3K3L0G3j0V

https://github.com/visionmedia/jade/commit/36addf44dff51569d0a3019b0a9f7f4577b3366a#diff-e1bbd4f15e3b63427b4261e05b948ea8R16