swiftlint: command not found - how can I fix this?

My environment is CircleCI, just so you know. and this is running as a Mac executor.

My `Dangerfile.swift` file looks like this:
```
import Danger
let danger = Danger()

let editedFiles = danger.git.modifiedFiles + danger.git.createdFiles
message("These files have changed: \(editedFiles.joined(separator: ", "))")

SwiftLint.lint()
```

Now, while Danger does write in my PR about modified and created files, `SwiftLint.lint()` run, and gives this error:
`/bin/sh: swiftlint: command not found`

Why would that be? The "danger-swift" Package contains SwiftLint as a dependency, so wouldn't it be able to run SwiftLint? Do I need to actually _install_ SwiftLint? If so, then what does the SwiftLint dependency do?