Update guzzlehttp/guzzle requirement to 6.3.3 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/master/CHANGELOG.md)
- [Commits](https://github.com/guzzle/guzzle/commits/6.3.3)

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

build/integration/composer.json

@@ -2,7 +2,7 @@
   "require-dev": {
     "phpunit/phpunit": "~4.6",
     "behat/behat": "^3.0",
-    "guzzlehttp/guzzle": "6.3.0",
+    "guzzlehttp/guzzle": "6.3.3",
     "jarnaiz/behat-junit-formatter": "^1.3",
     "sabre/dav": "3.2.2"
   }