Update Package.swift to Swift 5

[SamuelSchepp]

Hello, this commits makes GRMustache usable in a Swift 5 package.

Test this PR by creating a swift package like this:

mkdir host cd host swift package init

Package.swift:

// swift-tools-version:5.0

import PackageDescription

let package = Package(
    name: "host",
    products: [
        .library(
            name: "host",
            targets: ["host"]),
    ],
    dependencies: [
        .package(url: "https://github.com/SamuelSchepp/GRMustache.swift.git", .branch("master"))
    ],
    targets: [
        .target(
            name: "host",
            dependencies: ["Mustache"]),
        .testTarget(
            name: "hostTests",
            dependencies: ["host"]),
    ]
)

swift build