Added PSR-6 to Illuminate adapter (#140)

* Added PSR-6 to Illuminate adapter

* Required illuminate/cache 5.4+

* Drop PHP 5.5 support

Author: florianv

composer.json

@@ -19,7 +19,7 @@
         }
     ],
     "require": {
-        "php": "^5.5 || ^7.0",
+        "php": "^5.6 || ^7.0",
         "psr/cache": "^1.0",
         "psr/simple-cache": "^1.0"
     },