Remove link to sinatra-synchrony gem from README

Since the author won't be maintaining the sinatra-synchrony gem and recommends not using it, we probably shouldn't be linking to it.

From the sinatra-synchrony README:

> This gem should not be considered for a new application. It is better to use threads with Ruby,
> rather than EventMachine. It also tends to break when new releases of ruby come out, and EM itself
> is not maintained very well and has some pretty fundamental problems.
>
> I will not be maintaining this gem anymore. If anyone is interested in maintaining it, feel free
> to inquire, but I recommend not using EventMachine or sinatra-synchrony anymore.

README.md

@@ -57,7 +57,6 @@ Several higher-order Ruby projects have incorporated em-http and other Ruby HTTP
 - [Firering](https://github.com/EmmanuelOga/firering) - Eventmachine powered Campfire API
 - [RDaneel](https://github.com/hasmanydevelopers/RDaneel) - Ruby crawler which respects robots.txt
 - [em-eventsource](https://github.com/AF83/em-eventsource) - EventSource client for EventMachine
-- [sinatra-synchrony](https://github.com/kyledrake/sinatra-synchrony) - Sinatra plugin for synchronous use of EM
 - and many others.. drop me a link if you want yours included!
 
 ### License