Pin phpunit-json-assertions to v2

Author: mheap

composer.json

@@ -24,7 +24,7 @@
   "require-dev": {
     "phpunit/phpunit": "^5.7",
     "php-http/mock-client": "^0.3.0",
-    "estahn/phpunit-json-assertions": "@stable",
+    "estahn/phpunit-json-assertions": "^2.0.1",
     "squizlabs/php_codesniffer": "^3.1"
   },
   "autoload": {