Added getSubscriptionsCount function to KnockoutComputedFunctions

This function was missing and is expected to be available to all KnockoutComputer<T> objects, as stated in the docs:
http://knockoutjs.com/documentation/computedObservables.html#computed_observable_reference

knockout/knockout.d.ts

@@ -15,6 +15,7 @@ interface KnockoutSubscribableFunctions {
 
 interface KnockoutComputedFunctions extends KnockoutSubscribableFunctions {
     getDependenciesCount(): number;
+    getSubscriptionsCount(): number;
     hasWriteFunction(): boolean;
 }