Log Message:
Update ruby-kramdown to 1.8.0.

kramdown 1.8.0 released

This release contains only some minor updates and bug fixes.

Changes

* 4 minor changes:

  - The LaTeX converter now uses \texttt instead of \tt for code spans (fixes
    #257, reported by richard101696)

  - New option footnote_backlink for changing the backlink of footnotes in the
    HTML converter (fixes #247, requested by Benjamin Esham)

  - A quote directly followed by an ellipsis is now converted into an opening
    quotation mark (fixes #253, requested by Michael Franzl)

  - Removed warning for self-closing HTML elements that are not self-closed
    (fixes #262, requested by Gregory Pakosz)

* 3 bug fixes:

- Fixed #251: The special character sequence \` now works correctly when used
  in footnotes or headers that appear in the table of contents (reported by
  Peter Kehl)

- Fixed #254: kramdown crashed on encountering a table with multiple
  consecutive separator lines (reported by Christian Kruse)

- Fixed #256: Certain footnote definitions and codeblocks lead to crashes or
  unneeded backtracking in the regular expression engine - fixed by using
  atomic grouping (reported by Ali Ok)