parse.py: Unicode vs. str for Python 2

[CLiu13]

When running slimish-jinja via Python 2, I got this error:

 File "/home/travis/virtualenv/python2.7.14/lib/python2.7/site-packages/slimish_jinja/slimish_jinja.py", line 34, in preprocess
    Parser(lexer, callback=output.write, debug=self.environment.slim_debug).parse()
  File "/home/travis/virtualenv/python2.7.14/lib/python2.7/site-packages/slimish_jinja/parse.py", line 35, in parse
    self.callback(self.format_output(self.lookahead))
TypeError: unicode argument expected, got 'str'

I believe that this could be fixed by surrounding the return on parse.py:206 and parse.py:208 with unicode() (only when Python 2, since that method does not exist in Python 3), but I have not verified this yet.