Set latest PHPUnit 4.8.x version, because we use "--prefer-lowest" in Travis config.

Author: mekras

composer.json

@@ -23,7 +23,7 @@
     "require-dev": {
         "guzzlehttp/psr7": "^1.0",
         "php-http/client-integration-tests": "^0.5.1",
-        "phpunit/phpunit": "^4.8",
+        "phpunit/phpunit": "^4.8.27",
         "zendframework/zend-diactoros": "^1.0"
     },
     "autoload": {