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