Depend on hubot only development

`npm` loads the different hubot from the following:
* `./node_modules/hubot`
* `./node_modules/hubot-idobata/node_modules/hubot`

This cause instance mismatch using `instanceof`
* https://github.com/github/hubot/blob/v2.7.2/src/listener.coffee#L42

That's only `./node_modules/hubot` we need.
If `./node_modules/hubot-idobata/node_modules/hubot` is exist,
hubot-idobata wouldn't respond any message.

Fix #1
Merge pull request #5 from idobata/fix-incorrect-hubot-dependency

Depend on hubot only development