Futures 0.3.18 has been Yanked

[thomasmatecki]

See: rust-lang/futures-rs#2529

As a result futures = "0.3.18" is no longer available on crates.io.

$ cargo metadata
warning: please specify `--format-version` flag explicitly to avoid compatibility problems
    Updating git repository `https://github.com/rg3dengine/rg3d`
    Updating crates.io index
error: failed to select a version for the requirement `futures = "^0.3.18"`
candidate versions found which didn't match: 0.3.17, 0.3.16, 0.3.15, ...
location searched: crates.io index
required by package `rg3d-core v0.18.0 (https://github.com/rg3dengine/rg3d#ec581b75)`
    ... which satisfies git dependency `rg3d-core` of package `rg3d v0.24.0 (https://github.com/rg3dengine/rg3d#ec581b75)`
    ... which satisfies git dependency `rg3d` of package `game v0.1.0 (/home/thomas/Repositories/game)`