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

@@ -29,7 +29,7 @@
         }
     ],
     "require":     {
-        "php":                   "^5.5 || ^7.0",
+        "php":                   "^5.6 || ^7.0",
         "defuse/php-encryption": "^2.0",
         "cache/tag-interop":     "^1.0",
         "psr/cache":             "^1.0"