Add PHPUnit as a dev dependency.

App/composer.json

@@ -7,6 +7,9 @@
         "cakephp/framework": "dev-3.0",
         "php": ">=5.4"
     },
+    "require-dev": {
+        "phpunit/phpunit": "3.7.*"
+    }
     "autoload": {
         "psr-0": {
             "App\\": "."

lib/composer.json

@@ -20,6 +20,9 @@
     "require": {
         "php": ">=5.4"
     },
+    "require-dev": {
+        "phpunit/phpunit": "3.7.*"
+    }
     "autoload": {
         "psr-0": {
             "Cake\\": "Cake/"