Avoid encoding errors on compass filter

I was using webassets when I got this error on my Sass files.
Looks like it wasn't dealing well with non ascii characters.

    File "/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/StringIO.py", line 271, in getvalue
        self.buf += ''.join(self.buflist)
    UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 43793: ordinal not in range(128)