Initialize StubSpecification’s @name to make RubyGems warning-free

RubyGems 2.4.1 – which ships with Ruby 2.2.0-preview1 – is not warning-free:

`/home/chastell/.rubies/ruby-2.2.0-preview1/lib/ruby/2.2.0/rubygems/stub_specification.rb:158: warning: instance variable @name not initialized`

This makes the current master warning-free again.