PHPUnit 5.0 is required in order to mock methods with return type declarations

Author: Ocramius

composer.json

@@ -18,7 +18,7 @@
         "container-interop/container-interop": "~1.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "~4.6",
+        "phpunit/phpunit": "^4.6 || ^5.2.10",
         "ocramius/proxy-manager": "^1.0 || ^2.0",
         "squizlabs/php_codesniffer": "^2.0@dev",
         "phpbench/phpbench": "^0.10.0"