Add missing dependency on ext-json

Author: rudolfbyker

composer.json

@@ -14,7 +14,8 @@
         "php": ">=5.6",
         "ext-xml": "*",
         "ck/file_marc_reference": "^1.2",
-        "pear/file_marc": "^1.1"
+        "pear/file_marc": "^1.1",
+        "ext-json": "*"
     },
     "require-dev": {
         "phpunit/phpunit": "^5.7 | ^6.0 | ^7.0",