kramdown-parser-gfm was only pulled in transitively by solargraph, which
lives in the development group. Production installs therefore lacked the
GFM parser, so rendering any rich-text content page raised
"Kramdown::Error: kramdown has no parser to handle the specified input
format: GFM".
Declare kramdown-parser-gfm as a direct dependency and add a regression
spec for ApplicationHelper#markdown_to_html.
Fixes#264
raucao