Subcommand feature does not work on Ubuntu

Since `/usr/bin/which` rejects `-s` argument.
Reproducing steps on Ubuntu with SwiftLint:
```terminal.sh-session
$ docker run -it --rm norionomura/swiftlint:swift-4.2 bash
root@5d3cc0804b2c:/# echo "echo swiftlint-test executed">/usr/bin/swiftlint-test
root@5d3cc0804b2c:/# chmod +x /usr/bin/swiftlint-test
root@5d3cc0804b2c:/# swiftlint test
Illegal option -s
Usage: /usr/bin/which [-a] args
Unrecognized command: 'test'. See `swiftlint help`.
```

Start by reproducing the reported `swiftlint test` command in the Ubuntu Docker environment and inspect the subcommand implementation and its executable lookup. Done means the `test` subcommand runs successfully on Ubuntu without `/usr/bin/which` rejecting the `-s` argument.

Written by the indexing model from the issue text.