Remove async-rspec gem dependency.

Author: ioquatix

async.gemspec

@@ -23,7 +23,6 @@ Gem::Specification.new do |spec|
 	spec.add_dependency "io-event", "~> 1.1"
 	spec.add_dependency "timers", "~> 4.1"
 
-	spec.add_development_dependency "async-rspec", "~> 1.1"
 	spec.add_development_dependency "bake-test"
 	spec.add_development_dependency "bake-test-external"
 	spec.add_development_dependency "benchmark-ips"