Merge pull request #4 from rvalitov/bug/ext-json

[add] ext-json requirement

Author: coderholic

composer.json

@@ -18,7 +18,8 @@
     "require": {
         "php": "~5.6|~7.0",
         "guzzlehttp/guzzle": "^6.3",
-        "sabre/cache": "^1.0"
+        "sabre/cache": "^1.0",
+        "ext-json": "*"
     },
     "require-dev": {
         "phpunit/phpunit" : ">=5.4.3",