Update phpunit version

Christoph Rumpel · Christoph Rumpel · commit b1a21860c393 · 2017-06-13T20:28:34.000+02:00
diff --git a/composer.json b/composer.json
@@ -20,7 +20,7 @@
     },
     "require-dev": {
         "mockery/mockery": "^0.9.5",
-        "phpunit/phpunit": "~5.7",
+        "phpunit/phpunit": "5.*",
         "orchestra/testbench": "~3.3|~3.4"
     },
     "autoload": {
