Update the minimum PHPUnit version required

mnapoli · mnapoli · commit acee4611cade · 2017-04-09T00:15:05.000+02:00
diff --git a/composer.json b/composer.json
@@ -22,6 +22,6 @@
         "mindplay/composer-locator": "^2.1"
     },
     "require-dev": {
-        "phpunit/phpunit": "^5.0"
+        "phpunit/phpunit": "^5.7"
     }
 }
