Doesn't build on recommended branches/toolchains

[JellyWX]

PyO3 won't build on my Ubuntu 18.04 (Python 3.6 through APT)

Rustup 1.14.0

Attempted to build on 4 different Rustc versions, listed below:

Rustc STABLE 1.29.0 (What I had installed before I read- failed because of the features macro)
Rustc NIGHTLY 1.32.0 (Lastest nightly- failed with a Rust compiler error)
Rustc NIGHTLY 1.29 2018-07-16 (Version said to be lowest compatible on docs.rs)
Rustc NIGHTLY 1.30 2018-08-18 (Version said to be lowest compatible on the Readme- failed saying the version was too low. Rustup actually didn't have a version dating 2018-08-18 and installed the closest I presume which was 2018-08-17)

About to try some more versions I guess

[JellyWX]

Working on Rustc NIGHTLY 1.31 2018-10-09

[konstin]

The requirement was involuntarily bumped by rust-lang/rust#55380