Set new minimum PHPUnit 5.7 version

5.7.0 had a bug with how it matched data providers to the methods
utilizing them, particularly when traits were involved. Bumping the
miniumum version fixes the issue.

Author: weierophinney

composer.json

@@ -19,7 +19,7 @@
         "zendframework/zend-stdlib": "^2.7 || ^3.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^5.7||^6.0",
+        "phpunit/phpunit": "^5.7.15 || ^6.0.8",
         "zendframework/zend-authentication": "^2.5",
         "zendframework/zend-cache": "^2.6.1",
         "zendframework/zend-coding-standard": "~1.0.0",