Bump PHPUnit version to force compatibility with other libraries

Author: lornajane

composer.json

@@ -22,7 +22,7 @@
     "lcobucci/jwt": "^3.2"
   },
   "require-dev": {
-    "phpunit/phpunit": "^5.3",
+    "phpunit/phpunit": "^5.7",
     "php-http/mock-client": "^0.3.0",
     "estahn/phpunit-json-assertions": "@stable",
     "squizlabs/php_codesniffer": "^3.1"