fixed: UnicodeDecodeError during stemming with snowball stemmer

[log]
Traceback (most recent call last):
  File "stem_text.py", line 19, in <module>
    main()
  File "stem_text.py", line 14, in main
    stemming_result.append(stemmer.stem(word))
  File "/usr/local/lib/python2.7/site-packages/nltk/stem/snowball.py",
line 694, in stem
    word = (word.replace(u"\u2019", u"\x27")
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 5:
ordinal not in range(128)