Lock "debug" dependency to work on Node v4

Koa adds dependency to `"debug": "*"` which fetch a version non
compatible with Node v4. Looking it on our package.json fix this issue.

Author: Danilo Resende

package.json

@@ -47,6 +47,7 @@
     "koa": "^1.2.1",
     "koa-route": "^2.4.2",
     "koa-views": "^4.1.0",
+    "debug": "^2.6.1",
     "mocha": "^2.5.3",
     "pug": "^2.0.0-beta2",
     "redis": "^2.6.1",