Fix assertion for not existing files

Test fails with
```
$ mocha
  1) hubot:app creates files:
     AssertionError: bower.json, no such file or directory
      at /Users/yuuki/src/github.com/github/generator-hubot/node_modules/yeoman-generator/lib/test/assert.js:61:14
      at Array.forEach (native)
      at Function.assert.file (/Users/yuuki/src/github.com/github/generator-hubot/node_modules/yeoman-generator/lib/test/assert.js:59:10)
      at Context.<anonymous> (/Users/yuuki/src/github.com/github/generator-hubot/test/test-app.js:21:12)
```
So I fixed this.