Add missing required extensions to composer.json

Author: OndraM

composer.json

@@ -16,7 +16,8 @@
     "symfony/process": "^2.8 || ^3.1 || ^4.0",
     "ext-curl": "*",
     "ext-zip": "*",
-    "ext-mbstring": "*"
+    "ext-mbstring": "*",
+    "ext-json": "*"
   },
   "require-dev": {
     "phpunit/phpunit": "^5.7",
@@ -28,6 +29,9 @@
     "symfony/var-dumper": "^3.3 || ^4.0",
     "jakub-onderka/php-parallel-lint": "^0.9.2"
   },
+  "suggest": {
+    "ext-SimpleXML": "For Firefox profile creation"
+  },
   "autoload": {
     "psr-4": {
       "Facebook\\WebDriver\\": "lib/"