Update PHPUnit documentation (5.7, 4.8)

lib/docs/scrapers/phpunit.rb

@@ -24,7 +24,7 @@ module Docs
     HTML
 
     version '5' do
-      self.release = '5.5'
+      self.release = '5.7'
       self.base_url = "https://phpunit.de/manual/#{release}/en/"
     end