revert phpunit version specification

Author: mpackard@wayfair.com

composer.json

@@ -8,7 +8,7 @@
         "ramsey/uuid": "^3.5"
     },
     "require-dev": {
-        "phpunit/phpunit": "5.7"
+        "phpunit/phpunit": "^5.7"
     },
     "license": "BSD-2-Clause",
     "authors": [