Update plot-num-msg-by-time.py

from wechat.util import ......
from common.textutil import ensure_unicode

plot-num-msg-by-time.py

@@ -5,7 +5,7 @@
 # Author: Yuxin Wu <[email protected]>
 
 from wechat.parser import WeChatDBParser
-from wechat.utils import ensure_unicode
+from common.textutil import ensure_unicode
 
 from datetime import timedelta, datetime
 import numpy as np