Update Jade dependency.

Hi, I updated the Jade dependency to version `1.5.0`, as the previously used
version `0.35` was buggy.

For example, with the previous version, 

```jade
// layout.jade
doctype html
head
  block title
    title layout
```

```jade
block title
  title index
```

would yield `<title>layout</title>` instead of `<title>index</title>`.
This is fixed with the version update.

As `!!!` has been deprecrated, I replaced it by `doctype html` in Jade templates.