Use Subscription type instead of Subscribable

We claim compatibility with RxJS version 5 but in V5 the type Subscribable does not exist. This does not lead to an issue as the unsubscribe() method exists in Subscribable (V6+) and ISubscription (V5) but it is not very clean.

extensions/sherlock-rxjs/rxjs.ts

@@ -1,5 +1,5 @@
 import { _internal, atom, Derivable, ErrorWrapper, ReactorOptions } from '@politie/sherlock';
-import { Observable, Subscriber, Unsubscribable } from 'rxjs';
+import { Observable, Subscriber, Subscription } from 'rxjs';
 
 /**
  * Creates an RxJS Observable from a Derivable. Optionally accepts a `ReactorOptions` that governs RxJS emissions
@@ -20,7 +20,7 @@ export function toObservable<V>(derivable: Derivable<V>, options?: Partial<React
 export function fromObservable<V>(observable: Observable<V>): Derivable<V> {
     const atom$ = atom.unresolved<V>();
 
-    let subscription: Unsubscribable | undefined;
+    let subscription: Subscription | undefined;
     atom$.connected$.react(() => {
         if (atom$.connected && !subscription) {
             subscription = observable.subscribe({