Update to php-di 6

Changed

• Dependency to php-di is now ^6 which requires PHP7 @bastianschwarz
• Replaced Interop\Container\ContainerInterface with \Psr\Container\ContainerInterface in \de\codenamephp\platform\di\iContainer @bastianschwarz
• Changed constructor of \de\codenamephp\platform\di\Container to expect an instance of \DI\Definition\Source\MutableDefinitionSource and
  \Psr\Container\ContainerInterface as the parent constructor was changed @bastianschwarz
• All parameters of \de\codenamephp\platform\di\Container::__construct are now optional same as the parent constructor @bastianschwarz
• The type of the first parameter $name of \de\codenamephp\platform\di\Container::set is now enforced via type hint @bastianschwarz