Update test template to use hubot-mock-adapter

Greetings!

The purpose of this pull request is to update the test template to utilize the fancy [hubot-mock-adapter][1], which is a great way to thoroughly test your Hubot scripts! While I personally find the `hubot-mock-adapter` to be super useful, it takes a decent amount of boilerplate to get setup, so it's an ideal addition to the generator!

I also got sick of manually deleting the `test/temp` directory when re-running the tests, so I also added the [grunt-contrib-clean][2] plugin to the `test` task chain.

As always, let me know if you have any comments, questions or concerns! Thanks :smile: 

[1]: https://github.com/blalor/hubot-mock-adapter
[2]: https://github.com/gruntjs/grunt-contrib-clean