Update to latest rust nightly

The exact version is rustc 0.13.0-dev (0b7b4f075 2014-11-16 22:36:50 +0000) and the relevant upstream issues are rust-lang/rust#18752 and rust-lang/rust#18976.

Fixes #439.

I have tested this using huonw/compile_msg#7, but until it is merged the test suite won't compile without manually forking `compile_msg`.