[ruby/io-nonblock] Bump version to 0.2.0

https://github.com/ruby/io-nonblock/commit/46c0ec245e

ext/io/nonblock/io-nonblock.gemspec

@@ -1,6 +1,6 @@
 Gem::Specification.new do |spec|
   spec.name          = "io-nonblock"
-  spec.version       = "0.1.1"
+  spec.version       = "0.2.0"
   spec.authors       = ["Nobu Nakada"]
   spec.email         = ["nobu@ruby-lang.org"]