Fix ‘cargo metadata’ call for cargo 0.19.0-nightly

[andersk]

flycheck-rust-get-cargo-targets was failing in json-read because of a new warning emitted by cargo 0.19.0-nightly. Fix the warning, and also discard stderr so that future warnings don’t break things in the same way.

[fmdkdd]

Thanks for the fix 👍

Is --format-verison available on Rust 1.15 and up? (I'm confused by the version numbering of cargo vs. rustc).

[andersk]

Yes, it has been available since cargo metadata was first merged (rust-lang/cargo#2196).

$ cargo +1.15.1 version
cargo 0.16.0-nightly (6e0c18c 2017-01-27)
$ cargo +1.15.1 help metadata 
Output the resolved dependencies of a project, the concrete used versions
including overrides, in machine-readable format.

Usage:
    cargo metadata [options]

Options:
    -h, --help                 Print this message
    --features FEATURES        Space-separated list of features
    --all-features             Build all available features
    --no-default-features      Do not include the `default` feature
    --no-deps                  Output information only about the root package
                               and don't fetch dependencies.
    --manifest-path PATH       Path to the manifest
    --format-version VERSION   Format version [default: 1]
                               Valid values: 1
    -v, --verbose ...          Use verbose output (-vv very verbose/build.rs output)
    -q, --quiet                No output printed to stdout
    --color WHEN               Coloring: auto, always, never
    --frozen                   Require Cargo.lock and cache are up to date
    --locked                   Require Cargo.lock is up to date

[fmdkdd]

Hah! Nice trick there $ cargo +1.15.1 version for testing this :)

LGTM, thanks!