Add dependency on kramdown-parser-gfm
Add dependency on kramdown-parser-gfm

Refs https://github.com/thredded/thredded/issues/786
See https://github.com/gettalong/kramdown/blob/1035a20358b6c321bf4b5b3b0ba11f8388ca5565/doc/news/release_2_0_0.page#L50

jayroh/thredded,thredded/thredded,thredded/thredded,jayroh/thredded,thredded/thredded,jayroh/thredded,thredded/thredded,jayroh/thredded