Run tests on newer PHPUnit version (fixes #180)

Author: colinodell

composer.json

@@ -36,7 +36,7 @@
     },
     "require-dev": {
         "mikehaertl/php-shellcommand": "~1.1.0",
-        "phpunit/phpunit": "4.*",
+        "phpunit/phpunit": "^4.8|^5.7",
         "scrutinizer/ocular": "~1.1"
     },
     "bin": ["bin/html-to-markdown"],