About Swift Package Manager dependency issue.

In `Package.swift`, the `dependencies ` of `Cache` should not be `master`.

```
// swift-tools-version:5.1
import PackageDescription

let package = Package(
name: "Imaginary",
products: [
.library(
name: "Imaginary",
targets: ["Imaginary"]),
],
dependencies: [
// FIXME: it should not depend on `master` branch.
.package(url: "https://github.com/hyperoslo/Cache", .branch("master"))
],
targets: [
.target(
name: "Imaginary",
dependencies: ["Cache"],
path: "Sources"
)
],
swiftLanguageVersions: [.v5]
)
```