Update textproc/ruby-kramdown to 0.13.4.

Changes

* 1 minor change:

- Added a converter that extracts the TOC of a document (requested by
          Brendan Hay). Note that this is only useful if you use kramdown as a
          library!

* 7 bug fixes

- Fixed a typo: It should be --output and not --ouput (patch by
          postmodern)
- Fixed HTML converter to correctly output empty span tags (patch by
          John Croisant)
- Fixed bug RF#29350: Parsing of HTML tags with mismatched case now
          works
- Fixed bug RF#29426: Content of style tags is treated as raw text now
- HTML converter now uses rel instead of rev to be HTML5 compatible
          (patch by Joe Fiorini)
- Fixed Ruby 1.9.3 related warnings
- Fixed HTML parser to work around an implementation change of
          Array#delete_if in Ruby 1.9.3

(taca)