Update phpunit/phpunit requirement to ~6.5 in /build/integration

Updates the requirements on [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) to permit the latest version.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/master/ChangeLog-6.5.md)
- [Commits](https://github.com/sebastianbergmann/phpunit/commits/6.5.8)

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

build/integration/composer.json

@@ -1,6 +1,6 @@
 {
   "require-dev": {
-    "phpunit/phpunit": "~4.6",
+    "phpunit/phpunit": "~6.5",
     "behat/behat": "^3.0",
     "guzzlehttp/guzzle": "6.3.3",
     "jarnaiz/behat-junit-formatter": "^1.3",