README says crate requires --message-format cargo flag but doesn't use it

[CAD97]

Instead the crate appears to be scraping the output of cargo test --no-run --verbose.

See also roblabla/cargo-travis#34 (comment) where I showed how --message-format could be used for the purpose of gathering test binaries without scraping unstable human output. (Though I'm still not completely certain if cargo-test --message-format=json's output format should be considered stable.)