Fix for deprecated `!!!`

Fix for issue #17 as suggested by @francissta

Without this fix, running gfm and then navigating to a file gives an error on the console:

```
/usr/local/share/npm/lib/node_modules/gfms/views/layout.jade:1
  > 1| !!! 5
    2| html
    3|     head
    4|         title #{title} - Github Flavored Markdown Server

`!!!` is deprecated, you must now use `doctype` < /usr/local/share/npm/lib/$/gfms/$/jade/lib/lexer.js(246 < 821 < 111) < /usr/local/share/npm/lib/$/gfms/$/jade/lib/parser.js(109 < 86 < 124 < 138) < /usr/local/share/npm/lib/$/gfms/$/jade/lib/jade.js(95 < 152 < 256)
```