Fix confusion of the root-node when validating a signature - #424

Your requirements could not be resolved to an installable set of packages. Problem 1 - Root composer.json requires simplesamlphp/simplesamlphp ^3.0@dev -> satisfiable by simplesamlphp/simplesamlphp[3.0.x-dev (alias of dev-simplesamlphp-3.0)]. - simplesamlphp/simplesamlphp[dev-simplesamlphp-3.0, 3.0.x-dev] require simplesamlphp/saml2 dev-release-7.x -> satisfiable by simplesamlphp/saml2[dev-release-7.x] from composer repo (https://repo.packagist.org) but simplesamlphp/saml2 dev-bugfix/signature-root-confusion is the root package and cannot be modified. See https://getcomposer.org/dep-on-root for details and assistance. - simplesamlphp/simplesamlphp 3.0.x-dev is an alias of simplesamlphp/simplesamlphp dev-simplesamlphp-3.0 and thus requires it to be installed too.