Fix HTML detection

The old regexp didn't match DOCTYPE declarations like this:
~~~ html
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
~~~

More details here: https://github.com/shakyShane/grunt-browser-sync/issues/68#issuecomment-51725803