update gems (ruby2.1.5 and rails4.2.0.rc3) and add test

@smidwap 

Thank you for your great sample for Ruby on Rails using TodoMVC.
It is good sample to learn RoR.

I also want to support to learn RoR for someone who start studying RoR.
So I updated gem (ruby -> 2.1.5 and rails -> 4.2.0.rc3) and added rspecs.

If you don't mind please merge it.

## how to test

```
cd todomvc_on_rails
```

add qt for capybara-webkit
```bash
brew install qt
```

update gems
```bash
bundle update
```

execute test
```
rspec spec
```