Fix broken specialized implementations with Rust 1.40

[althonos]

Here are some things you should check for submitting your pull request:

• Run cargo fmt (This is checked by travis ci)
• Run cargo clippy and check there are no hard errors (There are a bunch of existing warnings; This is also checked by travis)
• If applicable, add an entry in the changelog.
• If applicable, add documentation to all new items and extend the guide.
• If applicable, add tests for all new or fixed functions
• If you changed any python code, run black .. You can install black with pip install black)

This PR hopefully resolves #614.

[konstin]

Thank you for fixing this so quickly!

@kngwyu Do you want to make a 0.8.1 release after that is merged?

[althonos]

No problem! Would be glad to have this released indeed.

[kngwyu]

Thank you for the fix!

@konstin
Yeah, I think we should.