Merge pull request #728 from localheinz/feature/phpunit

Enhancement: Require latest version of phpunit/phpunit

Author: SammyK

composer.json

@@ -16,7 +16,7 @@
         "paragonie/random_compat": "^2.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "~4.0",
+        "phpunit/phpunit": "^5.7.5",
         "mockery/mockery": "~0.8",
         "guzzlehttp/guzzle": "~5.0"
     },