bump phpunit

Author: AydinHassan

composer.json

@@ -34,7 +34,7 @@
     },
     "require-dev": {
         "composer/composer": "^1.2",
-        "phpunit/phpunit": "^5.6",
+        "phpunit/phpunit": "^5.7.2",
         "phpunit/phpunit-mock-objects": "^3.3",
         "squizlabs/php_codesniffer": "^2.4"
     },