Fix PHPUnit requirements

PHPUnit_Runner_Version::series() was present before, but has been fixed in sebastianbergmann/phpunit@3c4becb, which landed in the 4.8.25 and 5.3.3 release.

Author: kelunik

composer.json

@@ -7,7 +7,7 @@
         "php": ">=5.4.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^4.8.13|^5.1",
+        "phpunit/phpunit": "^4.8.25|^5.3.3",
         "jakub-onderka/php-parallel-lint": "^0.9.2",
         "jakub-onderka/php-console-highlighter": "^0.3.2"
     },