title: Use of undelcared type 'URLDataPromise'
body: <p>Currently updating an app from Swift 2 to Swift 4.2
  I am running into an error concerning URLDataPromise</p>
  
  <pre><code>extension NSURLConnection {
      public class func GET(URL: String, query: [NSObject:Any]? = nil) -&gt; URLDataPromise {
          return go(try OMGHTTPURLRQ.GET(URL, query))
      }
  
  }
  </code></pre>
  
  <p>The error in question is "Use of undeclared type 'URLDataPromise'
  The rest of the app imports PromiseKit 6 just fine.</p>