Fix import path for Padrino

Using Padrino (0.11.2), when using `@import "bootstrap";` I had an error:

    Syntax error: File to import not found or unreadable: bootstrap.
         Load paths:
             /my/stylesheet/path
             /my/project/vendor/assets/stylesheets

I had  `/my/project/vendor/assets/stylesheets` instead of `/path/to/bootstrap-sass/vendor/assets/stylesheets`.
Changing the way the Sass load path is generated fixed it.
This is also probably related to [Issue #354](https://github.com/thomas-mcdonald/bootstrap-sass/issues/354)