Update composer.json to allow PHP 7.

composer.json

@@ -22,7 +22,7 @@
         "files": ["lib/_autoload_modules.php"]
     },
     "require": {
-        "php": "~5.3",
+        "php": ">=5.3",
         "simplesamlphp/saml2": "~1.5.3",
         "simplesamlphp/xmlseclibs": "~1.3.2",
         "whitehat101/apr1-md5": "~1.0"