Restore php 5.4 dependency

Author: pitbulk

composer.json

@@ -16,7 +16,7 @@
         "source": "https://github.com/onelogin/php-saml/"
     },
     "require": {
-        "php": ">=5.6",
+        "php": ">= 5.4",
         "robrichards/xmlseclibs": "^3.0.0",
         "ext-openssl": "*",
         "ext-dom": "*"