Add logger dependency to gemspec

Author: kojix2

iruby.gemspec

@@ -22,6 +22,7 @@ Gem::Specification.new do |s|
   s.add_dependency 'data_uri', '~> 0.1'
   s.add_dependency 'ffi-rzmq'
   s.add_dependency 'irb'
+  s.add_dependency 'logger'
   s.add_dependency 'mime-types', '>= 3.3.1'
   s.add_dependency 'multi_json', '~> 1.11'
   s.add_dependency 'native-package-installer'