Update cleveref from 0.17.9 to 0.19

I'm trying to get the book to compile with htlatex so that we can have a
nice kindle-compatible epub, and it requires updating cleveref.
(Although I am a fan of MathML and the epubs for #287, e.g., of
@loopspace and @dvanduzer, Kindle doesn't currently support MathML at
all, and htlatex uses embedded images for the math.)