Use `<!doctype html>` instead of `<!DOCTYPE html>`

Return back to the lowercase doctype in order to be more consistent
with the lowercase of the html tags.

References:

  * h5bp/html5-boilerplate#1522
  * http://www.whatwg.org/specs/web-apps/current-work/multipage/syntax.html#the-doctype

Close h5bp/html5-boilerplate#1522.