Revert "Remove unnecessary constructor"

This reverts commit 97f15f7.

Still broken in Xcode 9.4.1 (which is currently the Travis default).

Sources/Property.swift

@@ -249,6 +249,13 @@ public class Property<TypeSpec: PropertyTypeSpec> {
 
 /// A property that can be set. Writes happen asynchronously.
 public class WriteableProperty<TypeSpec: PropertyTypeSpec>: Property<TypeSpec> {
+    // Due to a Swift bug I have to override this.
+    override init<Impl: PropertyDelegate, Notifier: PropertyNotifier>(
+        _ delegate: Impl, notifier: Notifier
+    ) where Impl.T == NonOptionalType {
+        super.init(delegate, notifier: notifier)
+    }
+
     /// The value of the property. Reading is instant and synchronous, but writing is asynchronous
     /// and the value will not be updated until the write is complete. Use `set` to retrieve a
     /// promise.