Added stable psr6

Author: Nyholm

composer.json

@@ -10,12 +10,6 @@
             "email": "[email protected]"
         }
     ],
-    "repositories": [
-        {
-            "type": "git",
-            "url":  "https://github.com/php-fig/cache.git"
-        }
-    ],
     "require": {
         "php": ">=5.4",
         "php-http/httplug": "^1.0@dev",
@@ -28,7 +22,7 @@
         "php-http/cookie": "^0.1@dev",
         "symfony/stopwatch": "^2.3",
         "psr/log": "^1.0",
-        "psr/cache": "dev-master",
+        "psr/cache": "1.0.0",
         "php-http/encoding": "^0.1@dev"
     },
     "autoload": {