Update the rust-toolchain pin from nightly 2018-10-01 to 2018-12-04.

The old pin was a nightly of 1.30.0.  The rand dependency pulls in the latest
0.2.44 version of libc.  (As does criterion in test mode.)  This version of libc
has a #[cfg] which ends up doing

    pub use core::ffi::c_void;

on most platforms (including OSX High Sierra and various *nix which I've tested
on).  However, libcore [stabilised the ffi module during 1.30.0](https://doc.rust-lang.org/nightly/src/core/ffi.rs.html#1)

causing the older nightly from a couple months ago to be unable to find it while
compiling libc.  This fixes the issue by updating the pinned nightly.  (This
will probably break some rustfmt things on CI, since @hdevalence said that's why
the pin was created in the first place.)

Author: isislovecruft

rust-toolchain

@@ -1 +1 @@
-nightly-2018-10-01
+nightly-2018-12-04