Update the dependent daru version

Author: mrkn

daru-io.gemspec

@@ -20,7 +20,7 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^bin/}) { |f| File.basename(f) }
   spec.require_paths = ['lib']
 
-  spec.add_development_dependency 'daru', '~> 0.1.5'
+  spec.add_development_dependency 'daru', '~> 0.2.0'
 
   spec.add_development_dependency 'bundler', '~> 1.15'
   spec.add_development_dependency 'rake', '~> 10.0'