Add support for Vapor 4

[mpdifran]

Is your feature request related to a problem? Please describe.
I'm updating to Vapor 4.x.x, and I'm currently blocked by this library.

because http >=3.0.0-rc.2.2 depends on swift-nio-ssl 1.0.1..<2.0.0 and websocket-kit >=2.0.0-beta.1 depends on swift-nio-ssl 2.0.0..<3.0.0, http is incompatible with websocket-kit.
And because vapor >=4.0.0 depends on websocket-kit 2.0.0..<3.0.0, http is incompatible with vapor.
And because root depends on vapor 4.3.0..<5.0.0 and root depends on http 3.0.0..<4.0.0, version solving failed.

Describe the solution you'd like
Update this library to work with Vapor 4.x.x

Additional context
Here's my Package.swift

// swift-tools-version:5.2
import PackageDescription

let package = Package(
    name: "API",
    platforms: [
        .macOS(.v10_15)
    ],
    products: [
        .library(name: "API",
                 targets: [
                    "App"
                 ]),
    ],
    dependencies: [
        .package(url: "https://github.com/vapor/vapor.git", from: "4.3.0"),
        .package(name: "HTTP", url: "https://github.com/vapor/http.git", from: "3.0.0"),
    ],
    targets: [
        .target(name: "App",
                dependencies: [
                    .product(name: "Vapor", package: "vapor"),
                    "HTTP"
                ]),
        .target(name: "Run",
                dependencies: [
                    "App"
                ]),
        .testTarget(name: "AppTests",
                    dependencies: [
                        "App"
                    ]),
    ]
)

[0xTim]

HTTP has been integrated into vapor/vapor for Vapor 4 so is no longer required (I should really archive it to be honest!)