Upgrade to nio4r 2.0.0

nio4r 2.0.0 primarily includes new features and bugfixes, with few breaking
changes. The primary reason for bumping the major version is dropping support
for all Ruby versions prior to 2.2.2, so as to match Rails 5.

Full release announcement here:

https://groups.google.com/forum/#!topic/socketry/ZDIUj1ufiJ8

Gemfile.lock

@@ -54,7 +54,7 @@ PATH
   specs:
     actioncable (5.1.0.alpha)
       actionpack (= 5.1.0.alpha)
-      nio4r (~> 1.2)
+      nio4r (~> 2.0)
       websocket-driver (~> 0.6.1)
     actionmailer (5.1.0.alpha)
       actionpack (= 5.1.0.alpha)
@@ -231,7 +231,7 @@ GEM
     mysql2 (0.4.5)
     mysql2 (0.4.5-x64-mingw32)
     mysql2 (0.4.5-x86-mingw32)
-    nio4r (1.2.1)
+    nio4r (2.0.0)
     nokogiri (1.7.0)
       mini_portile2 (~> 2.1.0)
     nokogiri (1.7.0-x64-mingw32)

actioncable/actioncable.gemspec

@@ -20,6 +20,6 @@ Gem::Specification.new do |s|
 
   s.add_dependency "actionpack", version
 
-  s.add_dependency "nio4r",            "~> 1.2"
+  s.add_dependency "nio4r",            "~> 2.0"
   s.add_dependency "websocket-driver", "~> 0.6.1"
 end