Make Racket doc cache message more prominent.

So people understand reason for the delay.

Also fix typo.
Fix issue #75 the right way.

The bug is actually about double-encoding. The root cause is that
read-html-as-xml should decode &amp; in attributes to & -- but it does
not do so. Fix _that_.

Also take the opportunity to define a new `read-html-as-xexprs` that can
be reused in a few places, instead of the awkwardness required to use
`read-html-as-xml` directly.

In conjunction with this, the markdown collection's `xexpr->string`
reverts to its previous behavior and now does encode & to &amp as of
v.015, so require that version in info.rkt.
Bump to v0.17
Add regression test for issue #75.
travis-racket v0.6