Update mdbook version

This version of mdbook includes
https://github.com/azerupi/mdBook/pull/207 , which is needed so that
we can start doing linkchecker on the various books.

This reverts commit 7f1d1c6.

The original commit was created because mdBook and rustdoc had
different generation algorithms for header links; now with
rust-lang#39966 , the algorithms
are the same. So let's undo this change.

... when I came across this problem, I said "eh, this isn't fun,
but it doesn't take that long." I probably should have just actually
taken the time to fix upstream, given that they were amenable. Oh
well!

Previously, mdBook used JavaScript to add header links, so we
skipped checking the book. As of
rust-lang#39966, it no longer does,
so we can start checking again.

There is a twist, though: it uses name instead of id, so let's test
for both. They're both valid links anyway, so it's good to have the
checker check anyway.

This reverts commit 7f1d1c6d9a7be5e427bace30e740b16b25f25c92.

The original commit was created because mdBook and rustdoc had
different generation algorithms for header links; now with
rust-lang/rust#39966 , the algorithms
are the same. So let's undo this change.

... when I came across this problem, I said "eh, this isn't fun,
but it doesn't take that long." I probably should have just actually
taken the time to fix upstream, given that they were amenable. Oh
well!