Error building the README usage example

[jshrake]

Building the README usage example produces the following error:

src/main.rs:11:32: 11:63 error: mismatched types:
 expected `*const std::os::raw::c_void`,
    found `*const ()`
(expected enum `std::os::raw::c_void`,
    found ()) [E0308]
src/main.rs:11         gl::load_with(|symbol| window.get_proc_address(symbol));
                                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/main.rs:11:32: 11:63 help: run `rustc --explain E0308` to see a detailed explanation
error: aborting due to previous error

I'm using rustc 1.6.0.