Update phpunit/phpunit requirement from 6.5 to 6.5.14 (#25)

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/6.5.14/ChangeLog-6.5.md)
- [Commits](sebastianbergmann/phpunit@6.5.0...6.5.14)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

composer.json

@@ -9,7 +9,7 @@
     "tightenco/collect": "^5.5"
   },
   "require-dev": {
-    "phpunit/phpunit": "6.5",
+    "phpunit/phpunit": "6.5.14",
     "brianium/paratest": "1.*"
   },
   "license": "MIT",