Fixed deprecation warning (spree_cmd)

 ```
spree extension extension_name
bundle install 
bundle exec rake test_app
bundle exec rspec spec
```

It will raise deprecation warning. 
The following is that reason.

+ RSpec::Core::ExampleGroup#example is deprecated and will be removed in RSpec 3
+ rspec-rails 3 will no longer automatically infer an example group's spec type from the file location.

[Gemfile.lock](https://gist.github.com/BlackPrincess/68135a6dd487c35a98ce)