Update phpunit

Author: vinkla

composer.json

@@ -21,7 +21,7 @@
         "php-http/guzzle6-adapter": "^1.1",
         "php-http/message": "^1.6",
         "php-http/mock-client": "^1.1",
-        "phpunit/phpunit": "^7.1"
+        "phpunit/phpunit": "^7.2"
     },
     "autoload": {
         "psr-4": {