Address recent change in Bundler

`Bundler::Definition#specs_for` no longer behaves as before. Admittedly
the previous behavior was more intuitive.

lib/warbler/traits/bundler.rb

@@ -127,7 +127,8 @@ def bundler_specs
         bundle_without = config.bundle_without.map { |s| s.to_sym }
         definition = ::Bundler.definition
         all = definition.specs.to_a
-        requested = definition.specs_for(definition.groups - bundle_without).to_a
+        dependencies = definition.dependencies_for(definition.groups - bundle_without)
+        requested = definition.send(:materialize, dependencies).to_a
         excluded_git_specs = (all - requested).select { |spec| ::Bundler::Source::Git === spec.source }
         excluded_git_specs.each { |spec| spec.groups << :warbler_excluded }
         requested + excluded_git_specs