Require the JSON extension as per new JSON methods

Author: Geolim4

composer.json

@@ -23,7 +23,8 @@
         "php": ">=5.5.0",
         "psr/cache": "1.0.x",
         "predis/predis": "1.0.x",
-        "phpfastcache/phpssdb": "0.0.x"
+        "phpfastcache/phpssdb": "0.0.x",
+        "ext-json": "*"
     },
     "suggest": {
         "ext-apc": "*",