Fix Rust 1.26 attribute regression

[toby]

From: rust-lang/rust#49934

Looks like Rust 1.26 breaks our build.

[chpio]

maybe im just blind, but where's the error: https://cargobomb-reports.s3.amazonaws.com/beta-1.26.0-beta.2/beta-2018-04-06/reg/serde_bencode-0.2.0/log.txt

current master compiles/tests just fine on current nightly (1.27.0-nightly (ad610bed8 2018-04-11))

[toby]

@chpio I also didn't parse an error out of that. I was going to build it with 1.26 after work today to see what was up. If nothing else, this ticket can be to go back and tell them there's no problem :)

[madadam]

Minimal rust version required by this crate is now 1.40.0. I'm closing this, but feel free to create a new issue if you need it to support older rusts.