fix pdf output by switching to absolute urls

There is a bug with the PDF output where assets with protocol-independent URLS such as `//cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.2.0/css/bootstrap.min.css` are not loaded https://github.com/jsonresume/resume-cli/issues/69
A workaround is to use the full URL.

For example, compare:
<http://registry.jsonresume.org/linuxbozo.pdf>
to:
<http://registry.jsonresume.org/linuxbozo.pdf?theme=kendall-fix>