Remove markdown_book_test as a gem dep
Remove markdown_book_test as a gem dep

Re: https://github.com/radar/twist/commit/d487ad10d36921fd3bd4a0f4d75547b9f3f6e132#commitcomment-5979057

This is actually a git submodule

RealSilo/twist,doug-c/mttwist,yannvery/twist,doug-c/mttwist,RealSilo/twist,radar/twist,radar/twist,doug-c/mttwist,radar/twist,yannvery/twist,yannvery/twist,RealSilo/twist