Breakage in nightly due to attributes in comments

[Dzmitry Malyshau]

See https://travis-ci.org/three-rs/three/jobs/394200787#L1965

error: `[cfg]` cannot be resolved, ignoring it...
 --> /home/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/cfg-if-0.1.3/src/lib.rs:1:28
  |
1 | //! A macro for defining #[cfg] if-else statements.
  |                            ^^^ cannot be resolved, ignoring