fix string interpolation when sys.maxunicode > 0x10000

Should hopefully fix the error in travis.
...
|     _char_tail = six.u('%s-%s' % (unichr(0x10000), unichr(min(sys.maxunicode, 0x10FFFF))))
|   File "/home/travis/build/xmlrunner/unittest-xml-reporting/six-1.7.3-py2.7.egg/six.py", line 589, in u
|     return unicode(s.replace(r'\\', r'\\\\'), "unicode_escape")
| TypeError: decoding Unicode is not supported