Update to latest stable phpunit

Author: richardmiller-zz

composer.json

@@ -19,7 +19,7 @@
         "guzzlehttp/cache-subscriber": "0.1.*"
     },
     "require-dev": {
-        "phpunit/phpunit": "4.3.*",
+        "phpunit/phpunit": "5.4.*",
         "phpdocumentor/phpdocumentor": "2.*",
         "predis/predis": "1.0.*"
     },