```ruby
lib = File.expand_path('../lib', __FILE__)
$LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
require 'rakuten_web_service/version'

Gem::Specification.new do |spec|
  spec.name          = 'rakuten_web_service'
  spec.version       = RakutenWebService::VERSION
  spec.authors       = ['Tatsuya Sato']
  spec.email         = ['tatsuya.b.sato@mail.rakuten.com']
  spec.description   = 'Ruby Client for Rakuten Web Service'
  spec.summary       = 'Ruby Client for Rakuten Web Service'
  spec.homepage      = 'http://webservice.rakuten.co.jp/'
  spec.license       = 'MIT'

  spec.files         = `git ls-files`.split($/)
  spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})
  spec.require_paths = ['lib']
  spec.required_ruby_version = '>= 2.3.0'

  spec.add_development_dependency 'bundler', '~> 1.3'
  spec.add_development_dependency 'rake', '~> 12.3.0'
  spec.add_development_dependency 'rspec', '~> 3.7.0'
  spec.add_development_dependency 'tapp', '~> 1.5.1'
  spec.add_development_dependency 'terminal-table', '~> 1.8.0'
  spec.add_development_dependency 'webmock', '~> 3.5.0'
end
```

Use the latest version of bundler
user
```ruby
lib = File.expand_path('../lib', __FILE__)
$LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
require 'rakuten_web_service/version'

Gem::Specification.new do |spec|
  spec.name          = 'rakuten_web_service'
  spec.version       = RakutenWebService::VERSION
  spec.authors       = ['Tatsuya Sato']
  spec.email         = ['tatsuya.b.sato@mail.rakuten.com']
  spec.description   = 'Ruby Client for Rakuten Web Service'
  spec.summary       = 'Ruby Client for Rakuten Web Service'
  spec.homepage      = 'http://webservice.rakuten.co.jp/'
  spec.license       = 'MIT'

  spec.files         = `git ls-files`.split($/)
  spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})
  spec.require_paths = ['lib']
  spec.required_ruby_version = '>= 2.3.0'

  spec.add_development_dependency 'bundler'
  spec.add_development_dependency 'rake', '~> 12.3.0'
  spec.add_development_dependency 'rspec', '~> 3.7.0'
  spec.add_development_dependency 'tapp', '~> 1.5.1'
  spec.add_development_dependency 'terminal-table', '~> 1.8.0'
  spec.add_development_dependency 'webmock', '~> 3.5.0'
end
```
assistant