Use Sass load path when Compass is not previously required

Hi,

This PR should fix the issue I reported in #327.

What was happening is that `compass` gem is installed on my machine so `require 'compass'` wasn't firing the `LoadError` and susy paths were added to `compass` (which wasn't used) instead of being included in the `SASS_PATH` environment variable.

With this fix, it now checks if Compass was required before, in this case, it loads `susy` paths in Compass, otherwise it adds it to `SASS_PATH`.

Works fine in local with `sass` compiler and `compass` compiler.

Let me know what you think,

Thanks!