Update RxJS reference (#480)

Previous RxJS link is potentially confusing. RxJS is now at v6, and can probably take care of describing it's own version history. Newcomers will mostly likely be looking for the latest version.

README.md

@@ -74,7 +74,7 @@ Flavors of Rx
 ---------------
 
 * __Rx.NET__: *(this repository)* The Reactive Extensions (Rx) is a library for composing asynchronous and event-based programs using observable sequences and LINQ-style query operators.
-* [RxJS 4](https://github.com/Reactive-Extensions/RxJS) / [RxJS 5 beta](https://github.com/ReactiveX/rxjs): The Reactive Extensions for JavaScript (RxJS) is a library for composing asynchronous and event-based programs using observable sequences and LINQ-style query operators in JavaScript which can target both the browser and Node.js.
+* [RxJS](https://github.com/ReactiveX/rxjs): The Reactive Extensions for JavaScript (RxJS) is a library for composing asynchronous and event-based programs using observable sequences and LINQ-style query operators in JavaScript which can target both the browser and Node.js.
 * [RxJava](https://github.com/ReactiveX/RxJava): Reactive Extensions for the JVM – a library for composing asynchronous and event-based programs using observable sequences for the Java VM.
 * [RxScala](https://github.com/ReactiveX/RxScala): Reactive Extensions for Scala – a library for composing asynchronous and event-based programs using observable sequences
 * [RxCpp](https://github.com/Reactive-Extensions/RxCpp): The Reactive Extensions for Native (RxCpp) is a library for composing asynchronous and event-based programs using observable sequences and LINQ-style query operators in both C and C++.