Update composer.json

Allow for PHPUnit 5.x

Author: padraic

Diff for: composer.json

@@ -13,7 +13,7 @@
     ],
     "require": {
         "php": ">=5.4.0",
-        "phpunit/phpunit": "~4.0"
+        "phpunit/phpunit": "^4.0|^5.0"
     },
     "autoload": {
         "psr-0": { "Humbug\\": "src/" }