[DRE-maint] Bug#972941: Bug#972941: jekyll: complains about missing kramdown-parser-gfm

[ydirson at free.fr]

> > The real problem is probably https://bugs.debian.org/972702. For
> > the
> > time being
> > please add
> > 
> > gem "kramdown-parser-gfm"
> > 
> > to your Gemfile and eventually remove Gemfile.lock. That should fix
> > it.
> 
> Yes that does help, thanks!

It is still probably worth mentionning that the ruby-bundler I have is 2.1.4-2,
not the 2.2.0~rc.1 mentionned in #972702.