Update phpunit to 7.5 (#29)

Author: slavcodev

composer.json

@@ -16,8 +16,8 @@
     "php": "^7.1",
     "ext-json": "*",
     "psr/http-message": "^1.0",
-    "phpunit/phpunit": "^6.0 | ^7.0",
-    "justinrainbow/json-schema": "~5.2"
+    "justinrainbow/json-schema": "~5.2",
+    "phpunit/phpunit": "~7.5"
   },
   "autoload": {
     "psr-4": {