Motivation for this change

Update cargo-make to the latest release.
https://github.com/sagiegurari/cargo-make/releases/tag/0.20.0

Note: doCheck is set as false.
Some tests fail because they need network access. However, Travis-CI ensures a proper build.
https://travis-ci.org/sagiegurari/cargo-make/jobs/546375769
https://travis-ci.org/sagiegurari/cargo-make/jobs/546375770
https://travis-ci.org/sagiegurari/cargo-make/jobs/546375771

cargo-make is a subcommand for cargo. You need to invoke cargo-make via cargo command.
To test the binary files in ./result/bin, consider the following commands:

[john@nixos:~/result/bin] PATH=.:$PATH cargo make --version
cargo-make 0.20.0
[john@nixos:~/result/bin] makers --version
makers 0.20.0

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.