Ask rubygems if the gemspec is supported on the current platform

RUBY_PLATFORM was i368-msys32, gemspec was x86-msys32
You're using the enumerator for lazy loading, right?  Otherwise it would be clearer to do:

```ruby
specs.collect do |s|
  Gem::Specification::load(File.join(app_root, spec_path))
end
```

I've never seen `Enumerator.new` used before so it took me a while to figure out why you weren't using the shorter `.collect` form :smile: