Convert Options struct into unit struct

[iKevinY]

Unit struct support was added in Docopt 0.8.1 via docopt/docopt.rs#217. Fixes #4174.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @matklad (or someone else) soon.

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[matklad]

I think Cargo.toml and Cargo.lock should be modified as well?

[iKevinY]

docopt was already at 0.8.1 in Cargo.lock, but I've gone and updated Cargo.toml.

[matklad]

Thanks!

@bors r+

[bors]

📌 Commit aa4f86f has been approved by matklad

[bors]

⌛ Testing commit aa4f86f with merge 88aa642...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: matklad
Pushing 88aa642 to master...