[enh] user agent string update

searx/utils.py

 import os
 import re
 
-ua_versions = ('26.0', '27.0', '28.0')
+ua_versions = ('29.0',
+               '30.0',
+               '31.0',
+               '32.0',
+               '33.0')
+
 ua_os = ('Windows NT 6.3; WOW64',
          'X11; Linux x86_64',
          'X11; Linux x86')
+
 ua = "Mozilla/5.0 ({os}) Gecko/20100101 Firefox/{version}"