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

@@ -23,7 +23,7 @@
         }
     ],
     "require":           {
-        "php":                  "^5.5 || ^7.0",
+        "php":                  "^5.6 || ^7.0",
         "psr/cache":            "^1.0",
         "cache/adapter-common": "^0.4"
     },