Log Message:
Update ruby-kramdown to 1.15.0.

kramdown 1.15.0 released (2017/09/08)

The most important change is that this release drops compatibility with Ruby
versions < 2.0!  Aside from that it includes some bug fixes, so updating is
recommended.

Changes

o 2 minor changes:

  * Drop compatibility with Ruby versions < 2.0
  * Make footnote output XML compatible (requested by Kriss Andsten)

o 4 bug fixes:

  * Apply abbreviation substitution to footnotes (fixes #440, reported by
    Thomas Leese)
  * Fix TOC entries by removing nested links (fixes #434, reported by Nate
    Wernimont)
  * Fix problems with brackets in certain situations when using the LaTeX
    converter (fixes #414, reported by Ryan Koppenhaver)
  * Fix escaping of URLs in the LaTeX converter (reported by Moshe Kamensky in
    pull request #67)

kramdown 1.14.0 released (2017/06/26)

This release adds support for rouge 2.x and fixes some minor issues - updating
is recommended.

Changes

o 2 minor changes:

* Adapt syntax highlighter for rouge to support version 1.x and 2.x (pull
  request #413 by Hirofumi Wakasugi)
* Alias Kramdown::Converter::HashAST to Kramdown::Converter::HashAst so that
  the convenience method on Kramdown::Document works (fixes #435, requested by
  Hirofumi Wakasugi)

o 1 bug fix:

* Fix problem with GFM header ID generation when << or >> is used \ 
(reported by
  Parker Moore)