Switch to latest PHPUnit for PHP 5 branch.

Author: sibprogrammer

composer.json

@@ -13,9 +13,10 @@
         "php": ">=5.6.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "5.5.*",
+        "phpunit/phpunit": "5.7.*",
         "phpunit/php-invoker": "*",
-        "phpunit/dbunit": ">=1.2"
+        "phpunit/dbunit": ">=1.2",
+        "sebastian/environment": "^2.0"
     },
     "config": {
         "process-timeout": 0