Update guzzlehttp/guzzle requirement in /build/integration

Updates the requirements on [guzzlehttp/guzzle](https://github.com/guzzle/guzzle) to permit the latest version.
- [Release notes](https://github.com/guzzle/guzzle/releases)
- [Changelog](https://github.com/guzzle/guzzle/blob/6.5.5/CHANGELOG.md)
- [Commits](https://github.com/guzzle/guzzle/compare/6.5.2...6.5.5)

---
updated-dependencies:
- dependency-name: guzzlehttp/guzzle
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

build/integration/composer.json

@@ -2,7 +2,7 @@
   "require-dev": {
     "phpunit/phpunit": "~6.5",
     "behat/behat": "~3.9.0",
-    "guzzlehttp/guzzle": "6.5.2",
+    "guzzlehttp/guzzle": "6.5.5",
     "jarnaiz/behat-junit-formatter": "^1.3",
     "sabre/dav": "3.2.3",
     "symfony/event-dispatcher": "~4.4"