Update pandoc on bots to get syntax highlighting in docs

[brson]

Pandoc now includes Rust syntax highlighting (#3264) so if we build a new pandoc from cabal on the doc bots we should have syntax highlighted docs. Yay!

Note: let's test this first to make sure rustdoc actually does the highlighting correctly.

[steveklabnik]

I think this is working now?

You just use

~~~ {.rust}
// some rust code
~~~

And it works out, I think.

[thestinger]

@steveklabnik: yup, it works locally with an up-to-date pandoc but the one used by the buildbot needs to be updated

[thestinger]

We're no longer using pandoc for rustdoc, so we'll need another way of doing syntax highlighting.