fix: update html-epub dependency
This is necessary, to get libtidy compiled on Node 10 (current LTS): https://github.com/gagern/node-libtidy/pull/25 html-epub!8 (merged)
This is necessary, to get libtidy compiled on Node 10 (current LTS): https://github.com/gagern/node-libtidy/pull/25 html-epub!8 (merged)