Matched travis' phpunit version

Author: grimzy

composer.json

@@ -16,7 +16,7 @@
         "jmikola/geojson": "^1.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^5.7",
+        "phpunit/phpunit": "5.6.5",
         "mockery/mockery": "^0.9.9",
         "laravel/laravel": "^5.2"
     },