Require PHPUnit 6.5

Currently the master branch of the server requires PHPUnit 6.5. The
Gallery tests use the TestCase class from the server, which in turn
extends the TestCase class from PHPUnit, so the PHPUnit version used in
the Gallery must match the one used in the server.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>

composer.json

@@ -28,7 +28,7 @@
   },
   "require-dev": {
     "guzzlehttp/guzzle": "^6.2",
-    "phpunit/phpunit": "^5.7|^6.0",
+    "phpunit/phpunit": "^6.5",
     "codeception/codeception": "2.3.*",
     "codeception/phpbuiltinserver": "*",
     "composer-plugin-api": "^1.0",