Update composer.json with 3.0.0 version of saml_auth so it works with PHP7.2

composer.json

@@ -390,7 +390,7 @@
         "jquery/toggles": "4.0.0",
         "jquery/word-and-character-counter": "2.5.1",
         "mikey179/vfsStream": "~1.2",
-        "onelogin/php-saml": "dev-master",
+        "onelogin/php-saml": "~3.0.0",
         "oomphinc/composer-installers-extender": "dev-master",
         "progress-tracker/progress-tracker": "v1.4.0",
         "phpunit/phpunit": ">=4.8.28 <5",