Composer constraint update

Author: wilr

composer.json

@@ -14,7 +14,7 @@
         "silverstripe/framework": "^4"
     },
     "require-dev": {
-        "phpunit/PHPUnit": "^5.7"
+        "phpunit/phpunit": "^5.7"
     },
     "autoload": {
         "psr-4": {