Add missing requires

These methods rescue a constant defined by `rubygems/remote_fetcher`,
so they should technically require it.

The require is provided by `gem_remote_fetcher` anyways but I was
running a unit spec that stubs that method, so I was getting an
undefined constant error hiding another error.

bundler/lib/bundler/rubygems_integration.rb

@@ -500,6 +500,7 @@ def plain_specs=(specs)
     end
 
     def fetch_specs(remote, name)
+      require "rubygems/remote_fetcher"
       path = remote.uri.to_s + "#{name}.#{Gem.marshal_version}.gz"
       fetcher = gem_remote_fetcher
       fetcher.headers = { "X-Gemfile-Source" => remote.original_uri.to_s } if remote.original_uri
@@ -518,6 +519,7 @@ def fetch_all_remote_specs(remote)
     end
 
     def download_gem(spec, uri, path)
+      require "rubygems/remote_fetcher"
       uri = Bundler.settings.mirror_for(uri)
       fetcher = gem_remote_fetcher
       fetcher.headers = { "X-Gemfile-Source" => spec.remote.original_uri.to_s } if spec.remote.original_uri