AppCenter CI support

Configured according to instructions: https://danger.systems/swift/guides/getting_started.html#setting-up-danger-to-run-on-your-ci

**Package.swift**
```
// swift-tools-version:4.2
import PackageDescription

let package = Package(
name: "AppCenter",
dependencies: [
.package(url: "https://github.com/danger/swift.git", from: "2.0.0")
],
targets: [
// This is just an arbitrary Swift file in our app, that has
// no dependencies outside of Foundation, the dependencies section
// ensures that the library for Danger gets build also.
.target(name: "AppCenter", dependencies: ["Danger"], path: "AppCenter", sources: ["Test.swift"]),
]
)
```

**Dangerfile.swift**
```
import Danger
let danger = Danger()

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

**appcenter-pre-build.sh**
```
cd $APPCENTER_SOURCE_DIRECTORY
npm install -g danger
swift build
swift run danger-swift ci
```

**AppCenter logs:**
```
nvm is not compatible with the npm config "prefix" option: currently set to "/usr/local"
Run `npm config delete prefix` or `nvm use --delete-prefix v6.17.1 --silent` to unset it.
/usr/local/lib/node_modules/danger/node_modules/ky/umd.js:59
returnValue = {...returnValue, [key]: value};
^^^

SyntaxError: Unexpected token ...
at createScript (vm.js:56:10)
at Object.runInThisContext (vm.js:97:10)
at Module._compile (module.js:549:28)
at Object.Module._extensions..js (module.js:586:10)
at Module.load (module.js:494:32)
at tryModuleLoad (module.js:453:12)
at Function.Module._load (module.js:445:3)
at Module.require (module.js:504:17)
at require (internal/module.js:20:19)
at Object. (/usr/local/lib/node_modules/danger/node_modules/ky-universal/index.js:34:5)
##[error]/bin/bash failed with return code: 1
##[error]Bash failed with error: /bin/bash failed with return code: 1
```
[7_Pre Build Script.txt](https://github.com/danger/swift/files/3490486/7_Pre.Build.Script.txt)

Any help? Thank you!