Fix Simple Form Foundation generator to use correct class with error

Right now, simple_form doesn't seem to _quite_ get the error class right for foundation. 

Before:
![before](https://f.cloud.github.com/assets/463175/2153597/b39fe6f0-9429-11e3-8f61-53d4834401d2.png)

After:
![after](https://f.cloud.github.com/assets/463175/2153598/b83f629e-9429-11e3-961a-52f081af2b6c.png)