Workaround for a known bug in gulp-sass.

gulp-sass will not compile .sass files without this option enabled. See https://github.com/dlmanning/gulp-sass/issues/55#issuecomment-47708591

>With default settings compiling sass files doesn't work. However there is a workaround. If you pass sourceComments: 'normal' as parameter the compilation work. The reason for this is that there is a strange condition which change how file are handled: https://github.com/dlmanning/gulp-sass/blob/master/index.js#L23-L27

**Reproduce**
Create a new .sass file with a simple test;
~~~
div
  margin: 10px
~~~
Add it to your gulpfile and run `gulp`.

It will error out with `Error: stdin:1: invalid top-level expression`