Ensure $? is not nil

[fukayatsu]

Hi,

I got an error on CircleCI like this.
Its parallelism was 2x.

bundle exec rspec --color --require spec_helper --format=progress --format=Nc ...
...

Finished in 1 minute 4.69 seconds (files took 5.44 seconds to load)
228 examples, 0 failures
terminal-notifier is only supported on Mac OS X 10.8, or higher.


/home/ubuntu/FooProject/vendor/bundle/ruby/2.2.0/gems/terminal-notifier-1.6.2/lib/terminal-notifier.rb:53:in `notify': undefined method `success?' for nil:NilClass (NoMethodError)
    from /home/ubuntu/FooProject/vendor/bundle/ruby/2.2.0/gems/rspec-nc-0.2.0/lib/nc.rb:47:in `say'
    from /home/ubuntu/FooProject/vendor/bundle/ruby/2.2.0/gems/rspec-nc-0.2.0/lib/nc.rb:37:in `dump_summary'
    from /home/ubuntu/.rvm/gems/ruby-2.2.0@global/gems/rspec-core-3.2.1/lib/rspec/core/reporter.rb:146:in `block in notify'
    from /home/ubuntu/.rvm/gems/ruby-2.2.0@global/gems/rspec-core-3.2.1/lib/rspec/core/reporter.rb:145:in `each'
    from /home/ubuntu/.rvm/gems/ruby-2.2.0@global/gems/rspec-core-3.2.1/lib/rspec/core/reporter.rb:145:in `notify'
    from /home/ubuntu/.rvm/gems/ruby-2.2.0@global/gems/rspec-core-3.2.1/lib/rspec/core/reporter.rb:130:in `finish'
    from /home/ubuntu/.rvm/gems/ruby-2.2.0@global/gems/rspec-core-3.2.1/lib/rspec/core/reporter.rb:64:in `report'
    from /home/ubuntu/.rvm/gems/ruby-2.2.0@global/gems/rspec-core-3.2.1/lib/rspec/core/runner.rb:108:in `run_specs'
    from /home/ubuntu/.rvm/gems/ruby-2.2.0@global/gems/rspec-core-3.2.1/lib/rspec/core/runner.rb:86:in `run'
    from /home/ubuntu/.rvm/gems/ruby-2.2.0@global/gems/rspec-core-3.2.1/lib/rspec/core/runner.rb:70:in `run'
    from /home/ubuntu/.rvm/gems/ruby-2.2.0@global/gems/rspec-core-3.2.1/lib/rspec/core/runner.rb:38:in `invoke'
    from /home/ubuntu/.rvm/gems/ruby-2.2.0@global/gems/rspec-core-3.2.1/exe/rspec:4:in `<top (required)>'
    from /home/ubuntu/FooProject/vendor/bundle/ruby/2.2.0/bin/rspec:23:in `load'

[julienXX]

Thanks!