Merge pull request #15 from meganemura/improve-gemspec-and-gemfile

Move gem specifications from Gemfile to gemspec
Remove old rubocop configurations
Generate `.rubocop_todo.yml`

$ rubocop --auto-gen-config
Add .rubocop.yml
Clean Rakefile
RuboCop: Style/HashSyntax: EnforcedStyle: hash_rockets
RuboCop: Enable Lint/UnusedBlockArgument
RuboCop: Enable Lint/UnusedMethodArgument