Bump required pandoc version to 1.9.

Earlier versions of pandoc don't have the default.html5 and thus,
building the docs fail.

Signed-off-by: Noufal Ibrahim <noufal@nibrahim.net.in>
auto merge of #10222 : nibrahim/rust/docfix, r=brson

Earlier versions of pandoc don't have the `default.html5` template file. When `make docs` is run, the build process fails with this message.

    pandoc: doc/rust.html
    pandoc: /usr/share/pandoc-1.8.2.1/templates/default.html5: openFile: does not exist (No such file or directory)
    
    node.js:201
            throw e; // process.nextTick error, or 'error' event on first tick
                  ^
    Error: write EPIPE
        at errnoException (net.js:670:11)
        at Object.afterWrite [as oncomplete] (net.js:503:19)
    make: *** [doc/rust.html] Error 1