chore: add support for PHPUnit 8 (#5252)

Author: bshaffer

composer.json

@@ -26,7 +26,8 @@
  "google/gax": "^1.6.0"
  },
  "require-dev": {
- "phpunit/phpunit": "^4.8|^5.0",
+ "phpunit/phpunit": "^4.8|^5.0|^8.0",
+ "yoast/phpunit-polyfills": "^1.0",
  "phpdocumentor/reflection": "^3.0"
  },
  "suggest": {