Log Message:
markdown-mode: update to 2.4.

Change MASTER_SITE to author's github.

Breaking changes:
  GNU Emacs 25.1 or later is required. And xemacs support has been dropped
  Face variables, such as markdown-italic-face are now
  obsolete. Use face names directly in code and customizations.
  The face names themselves are unaffected, so this shouldn't
  affect most users.
  Delete obsoleted aliases
  Internal variables markdown-font-lock-keywords-basic and
  gfm-font-lock-keywords are now obsolete.
  markdown-font-lock-keywords is now used instead, but users
  should use font-lock-add-keywords instead of modifying this
  variable.
  markdown-mode now adds entries to the beginning of
  auto-mode-alist rather than the end. If you were relying on
  the previous behavior in order to override these entries, you
  should fix the problem by following best practice and ensuring
  that your user configuration is loaded after the autoloads for
  markdown-mode are evaluated. (GH-331, GH-335)
  Point at the end of fenced code blocks is no-longer considered
  part of the code block ([GH-#349][]).
  Enable markdown-fontify-code-blocks-natively in gfm-view-mode. ([GH-#451][])

New features:
  GFM task list item (checkbox) insertion with C-c C-s [, or
  as a final fallback for markdown-do (C-c C-d). Thanks to
  Akinori Musha for a patch. (GH-229)
  Optionally move leading atx heading markup to the left margin
  when markdown-marginalize-headers is non-nil. Thanks to
  Alexis Gallagher for a patch. (GH-272, GH-274)
  Added pipe table editing features. Thanks to Dmitry Safronov
  for a patch. (GH-171, GH-266)
  Font lock for HTML tags and attributes, with new faces
  markdown-html-tag-name-face,
  markdown-html-tag-delimiter-face,
  markdown-html-attr-name-face, and
  markdown-html-attr-value-face. (GH-249)
  Font lock for HTML entities, with a new face
  markdown-html-entity-face.
  Scale down large inline images using markdown-max-image-size,
  a cons cell of the form (max-width . max-height).
  Added read-only viewing modes markdown-view-mode and
  gfm-view-mode with keymaps similar to view-mode and
  help-mode. (GH-296)
  Optionally add footnote definitions to the end of the imenu
  index using markdown-add-footnotes-to-imenu. (GH-235)
  Add custom variables markdown-xhtml-body-preamble and
  markdown-xhtml-body-epilogue for wrapping additional XHTML
  tags around the output. (GH-280, GH-281)
  Add markdown-unused-refs command to list and clean up unused
  references (available via C-c C-c u). (GH-322)
  Add markdown-insert-table (C-c C-s t) for interactive
  table insertion. (GH-369)
  Add markdown-kill-outline and markdown-kill-block functions.
  Added markdown-display-remote-images for viewing remote
  images. Thanks to Sean Allread for the patch. (GH-378)
  markdown-back-to-heading can be used as command (GH-415)