Merge pull request #12 from sokac/fix/unittest

Fix phpunit version

Author: whitemerry

composer.json

@@ -15,7 +15,7 @@
         "php": ">=5.3.0"
     },
     "require-dev": {
-        "phpunit/phpunit": ">=5.7"
+        "phpunit/phpunit": "~5.7"
     },
     "autoload": {
         "psr-4": {