devel/ruby-async: add package version 2.2.1

Required by ruby-html-proofer package.

Async

Async is a composable asynchronous I/O framework for Ruby based on io-event
and timers.

    "Lately I've been looking into async, as one of my projects -
    tus-ruby-server - would really benefit from non-blocking I/O.
    It's really beautifully designed." - janko

Features

* Scalable event-driven I/O for Ruby. Thousands of clients per process!
* Light weight fiber-based concurrency. No need for callbacks!
* Multi-thread/process containers for parallelism.
* Growing eco-system of event-driven components.

(taca)