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.
Merge pull request #265 from pje/patch-1

Remove link to sinatra-synchrony gem from README