specify dependencies properly.

I tried to install this gem and run it as per the instructions in the README but quickly got an error that said:

```shell
`require': cannot load such file -- curses (LoadError)
```

So i took a peek at the gemspec and noticed that curses dependency was specified incorrectly. I couldn't find a "add_dependency" configuration in the Specification Reference but there is an [add_runtime_dependency](http://guides.rubygems.org/specification-reference/#add_runtime_dependency).