Update cargo dependencies

[sadikovi]

This small PR updates dependencies to the latest versions.

[sadikovi]

@sunchao Do you think that we should use ~ to restrict versions in Cargo.toml (https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html)?

[coveralls]

Pull Request Test Coverage Report for Build 621

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 95.551%

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Totals
Change from base Build 619:	0.0%
Covered Lines:	12415
Relevant Lines:	12993

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💛 - Coveralls

[sunchao]

Thanks @sadikovi . The PR looks good to me. Regarding the "tilde" versions, I think we may not need it for now as #160 may just be a Rust nightly upgrade issue. Also, I checked some other major Rust projects and they all use the caret versions.