Add PHPUnit as dev dependency

Author: stil

composer.json

@@ -6,10 +6,13 @@
         "php": ">=5.3",
         "ext-gd": "*"
     },
+    "require-dev": {
+        "phpunit/phpunit": "^5.4"
+    },
     "autoload": {
         "psr-4": {
             "GDText\\": "src/",
             "GDText\\Tests\\": "tests/"
         }
     }
-}
+}