Allow ~6.4

Author: gareth-ib

Diff for: composer.json

@@ -64,7 +64,7 @@
         "behat/behat": "^3.0",
         "mikey179/vfsStream": "^1.2",
         "mockery/mockery": "^1.0",
-        "phpunit/phpunit": "^6.4",
+        "phpunit/phpunit": "~6.4",
         "squizlabs/php_codesniffer": "^1.4",
         "symfony/dotenv": "^4.0"
     },