Add PHPUnit dev dependency.

composer.json

@@ -15,6 +15,9 @@
     "require": {
         "php": ">=5.2.4"
     },
+    "require-dev": {
+        "phpunit/phpunit": "*"
+    },
     "autoload": {
         "psr-0": { "Mustache": "src/" }
     }