Fix _settings.scss filepath

The generator is currently broken under rails 4:
```
rails g foundation:install
```
displays the error

```
gems/foundation-rails-5.0.3.0/lib/foundation/rails/generators/install_generator.rb:19:in `read': No such file or directory - gems/foundation-rails-5.0.3.0/lib/foundation/rails/generators/../../../../vendor/_settings.scss (Errno::ENOENT)
```

This pull request fixes the path that the generator is using.

Why is there no issue tracker for this repository?