remove broken link (ruby implementation) (#1823)

README.md

@@ -105,7 +105,6 @@ Flavors of Rx
 * [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++.
-* [Rx.rb](http://rxrb.codeplex.com): A prototype implementation of Reactive Extensions for Ruby (Rx.rb).
 * [RxPy](https://github.com/ReactiveX/RxPY): The Reactive Extensions for Python 3 (Rx.Py) is a set of libraries to compose asynchronous and event-based programs using observable collections and LINQ-style query operators in Python 3.