Fix tests.

Author: greg-md

Dockerfile

@@ -0,0 +1,6 @@
+FROM php:7.2-cli
+
+RUN pecl install xdebug-2.6.0 \
+    && docker-php-ext-enable xdebug
+
+WORKDIR /var/www/app

composer.json

@@ -18,8 +18,8 @@
     "ext-json": "*"
   },
   "require-dev": {
-    "phpunit/phpunit": "6.0.*",
-    "symfony/var-dumper": "^3.2"
+    "phpunit/phpunit": "^7.4",
+    "symfony/var-dumper": "^4.1"
   },
   "autoload": {
     "psr-4": {