Fix missing ntwitter module in README.md

Hubot raised an error when executing README.md's install instraction.
```
$ hubot -a kandan
npm WARN package.json optparse@1.0.3 No repository field.
...
[Fri Nov 07 2014 23:16:48 GMT+0900 (JST)] ERROR Unable to load /Users/yuuki/docker/hubot/node_modules/hubot-scripts/src/scripts/tweet: Error: Cannot find module 'ntwitter'
```
It is caused by missing dependancy for ntwitter.