Add PromiseKit dependency

- Added PromiseKit dependency

Carthage/Checkouts/PromiseKit/Extensions/MapKit/Sources/MKMapSnapshotter+Promise.swift

@@ -0,0 +1,28 @@
+import MapKit
+#if !PMKCocoaPods
+import PromiseKit
+#endif
+
+/**
+ To import the `MKMapSnapshotter` category:
+
+    use_frameworks!
+    pod "PromiseKit/MapKit"
+
+ And then in your sources:
+
+    import PromiseKit
+*/
+extension MKMapSnapshotter {
+#if swift(>=4.2)
+    /// Starts generating the snapshot using the options set in this object.
+    public func start() -> Promise<Snapshot> {
+        return Promise { start(completionHandler: $0.resolve) }
+    }
+#else
+    /// Starts generating the snapshot using the options set in this object.
+    public func start() -> Promise<MKMapSnapshot> {
+        return Promise { start(completionHandler: $0.resolve) }
+    }
+#endif
+}