RuboCop was introduced by #226 2 years ago, but there are some missing settings.
In this PR, I would like to change the followings so that we can check code easily and correctly.

• Add rubocop to development_dependency in zipruby.gemspec for developer friendliness
• Set TargetRubyVersion of RuboCop to 1.9, which is the minimum required Ruby version for rubyzip.
  • https://github.com/bbatsov/rubocop/blob/master/manual/configuration.md#setting-the-target-ruby-version
• Disable Style/MutableConstant because some existent code relies on it
  • http://www.rubydoc.info/gems/rubocop/RuboCop/Cop/Style/MutableConstant
• Apply rubocop changes by bundle exec rubocop -a

If we are not satisfied with default rubocop style, we can modify the rules at .rubocop.yml afterward.