SourceMaps: Don't fix map generated by upstream

This fixes the tests for both Windows and Linux; when https://github.com/sass/libsass/pull/477 is applied.

Note: the sourcemap filename should be the complete path when using node-sass from downstream.

Note 2: On Windows, however, MSVCR throws exception on `npm install` but continues. `node-gyp rebuild` runs just fine. After that its all ok.