Uses Pathname#join rather than File#join when possible

spec/app/finders/themes/urls_in_homepage_spec.rb

@@ -2,7 +2,7 @@ describe WPScan::Finders::Themes::UrlsInHomepage do
   subject(:finder) { described_class.new(target) }
   let(:target)     { WPScan::Target.new(url) }
   let(:url)        { 'http://wp.lab/' }
-  let(:fixtures)   { File.join(FINDERS_FIXTURES, 'themes', 'urls_in_homepage') }
+  let(:fixtures)   { FINDERS_FIXTURES.join('themes', 'urls_in_homepage') }
 
   it_behaves_like 'App::Finders::WpItems::URLsInHomepage' do
     let(:type)                { 'themes' }