Force SASS to use UTF-8, even on Windows

See http://blog.pixelastic.com/2014/09/06/compass-utf-8-encoding-on-windows/

SASS uses Ruby's default character set if it can't find either a UTF-8 BOM (eww) or a `@charset` line right at the top of each .scss file (equally eww). On Linux/Mac, Ruby's default charset is UTF-8 so that isn't a problem, but on Windows it uses CP1252 for reading - and then writes in UTF-8.

End result is a file where all non-ASCII characters are completely corrupted.

As the linked article says, the best way to fix this is to force SASS to use UTF-8 by passing it the `-Eutf-8` command-line argument.

Obviously the ideal solution here would be to allow users of `gulp-ruby-sass` to specify their own charset, but this is at least good enough to fix the Windows bug...