nightly builds on CI broken

[fiveop]

The nightly builds for PRs fail with the error message:

the crate X has been compiled with rustc 1.10.0-nightly (576229fea 2016-04-15), which is incompatible with this version of rustc [E0514]

where X in {std, core, and a few others}. The failures are not caused by changes due to the PRs, since the first failure (https://travis-ci.org/rust-lang/libc/builds/123657547) happens for a patch that just changes the README.md

[alexcrichton]

Thanks for the report! Attempting to fix in #267, we'll see how that turns out.

[alexcrichton]

Ok, fixed in #267