title: Xcode 10 beta SnapKit constraints type error
body: <p>I'm using the Xcode 10 beta 5. My project is running on Swift 4 with the 4.0.0 SnapKit. 
  I have a function which sets up <code>UITableView</code>'s constraints:</p>
  
  <pre><code>private func setupConstraints() {
      tableView.snp.makeConstraints { make in
          make.leading.equalTo(view.safeAreaLayoutGuide.leadingAnchor)
          make.trailing.equalTo(view.safeAreaLayoutGuide.trailingAnchor)
          make.top.equalTo(view.safeAreaLayoutGuide.topAnchor)
          make.bottom.equalTo(view.safeAreaLayoutGuide.bottomAnchor)
      }
  }
  </code></pre>
  
  <p>This function produces the following errors:
  <code>Argument type 'NSLayoutXAxisAnchor' does not conform to expected type 'ConstraintRelatableTarget'</code></p>
  
  <p>I tried <code>view.safeAreaLayoutGuide.leadingAnchor as! ConstraintRelatableTarget</code>, but this casting fails.</p>
  
  <p>Do you guys have any idea how to fix it?</p>