External adapters exposed

  - this was inadvertently removed in 0c873c1
  - the feature was introduced in #941

faraday.gemspec

@@ -22,6 +22,7 @@ Gem::Specification.new do |spec|
 
   files = %w[CHANGELOG.md LICENSE.md README.md Rakefile examples lib spec]
   spec.files = `git ls-files -z #{files.join(' ')}`.split("\0")
+  spec.require_paths = %w[lib spec/external_adapters]
   spec.metadata = {
     'homepage_uri' => 'https://lostisland.github.io/faraday',
     'changelog_uri' =>