downgrade phpunit to support 7.0 in CI

Author: aivis

composer.json

@@ -15,7 +15,7 @@
         "illuminate/support": "~5.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "~7.0",
+        "phpunit/phpunit": "~6.0",
         "mockery/mockery": "^0.9.4",
         "orchestra/testbench": "^3.5"
     },