Leaf 4.0.0 Beta 2

• Updated to latest LeafKit and Vapor beta 2 releases (#149)

Configuration changes:

import Leaf
import Vapor

// beta.1
app.use(LeafProvider())

// beta.2
app.views.use(.leaf)

Rendering changes:

// beta.2
app.get("test-file") { req in
    req.view.render(#file, ["foo": "bar"])
}