Fix build on nightly

[petrochenkov]

Hi.

rust-lang/rust#42894 makes some compatibility lints in rustc deny-by-default and regression testing found that this crate is affected. Here's a patch fixing the deprecated code (see rust-lang/rust#34537 for more information about the issue).

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[petrochenkov]

@hobofan
Closing due to inactivity.
The crate still doesn't build on nightly.