Correct path to _settings file.

I attempted to run `rails generate foundation:install` on a simple rails app, and hit an error:

```
> rails generate foundation:install
      insert  app/assets/javascripts/application.js
      append  app/assets/javascripts/application.js
/Users/brookr/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/foundation-rails-5.0.3.0/lib/foundation/rails/generators/install_generator.rb:19:in `read': No such file or directory @ rb_sysopen - /Users/brookr/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/foundation-rails-5.0.3.0/lib/foundation/rails/generators/../../../../vendor/_settings.scss (Errno::ENOENT)
	from /Users/brookr/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/foundation-rails-5.0.3.0/lib/foundation/rails/generators/install_generator.rb:19:in `add_assets'
	from /Users/brookr/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/thor-0.18.1/lib/thor/command.rb:27:in `run'
	from /Users/brookr/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/thor-0.18.1/lib/thor/invocation.rb:120:in `invoke_command'
	from /Users/brookr/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/thor-0.18.1/lib/thor/invocation.rb:127:in `block in invoke_all'
	from /Users/brookr/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/thor-0.18.1/lib/thor/invocation.rb:127:in `each'
	from /Users/brookr/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/thor-0.18.1/lib/thor/invocation.rb:127:in `map'
	from /Users/brookr/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/thor-0.18.1/lib/thor/invocation.rb:127:in `invoke_all'
	from /Users/brookr/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/thor-0.18.1/lib/thor/group.rb:233:in `dispatch'
	from /Users/brookr/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/thor-0.18.1/lib/thor/base.rb:439:in `start'
	from /Users/brookr/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/railties-4.0.2/lib/rails/generators.rb:156:in `invoke'
	from /Users/brookr/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/railties-4.0.2/lib/rails/commands/generate.rb:11:in `<top (required)>'
	from /Users/brookr/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/activesupport-4.0.2/lib/active_support/dependencies.rb:229:in `require'
	from /Users/brookr/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/activesupport-4.0.2/lib/active_support/dependencies.rb:229:in `block in require'
	from /Users/brookr/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/activesupport-4.0.2/lib/active_support/dependencies.rb:214:in `load_dependency'
	from /Users/brookr/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/activesupport-4.0.2/lib/active_support/dependencies.rb:229:in `require'
	from /Users/brookr/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/railties-4.0.2/lib/rails/commands.rb:48:in `<top (required)>'
	from bin/rails:4:in `require'
	from bin/rails:4:in `<main>'
```

It seems the `_settings.scss` is actually in a different place than expected. 

This PR presumes the path is incorrect, and the file is in the right place. 

I'd like to submit a test for this as well, but I'm not sure how this repo would like tests organized. Give me some hints, and I'll see what I can do.

Thanks!