Fixed one more issue with unicode text

django-bootstrap3 does not properly handle unicode chars as form field values.

`File ".../ve/local/lib/python2.7/site-packages/bootstrap3/forms.py", line 192, in render_field_and_label
  html = render_label(label, label_class=label_class) + html
UnicodeDecodeError: ‘ascii’ codec can't decode byte 0xc3 in position 224: ordinal not in range(128)`