Fix an error for Gemspec/DependencyVersion

[koic]

This PR fixes the following error for Gemspec/DependencyVersion when add_dependency without version argument and method called on gem name argument:

Gem::Specification.new do |spec|
  spec.add_dependency('parser'.freeze)
end

$ bundle exec rubocop --only Gemspec/DependencyVersion -d
(snip)

undefined method `value' for s(:send,
  s(:str, "parser"), :freeze):RuboCop::AST::SendNode
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/gemspec/dependency_version.rb:97:in `allowed_gem?'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/gemspec/dependency_version.rb:84:in `on_send'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/commissioner.rb:143:in `public_send'

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