Corrected format string to support python 2.6 (< 2.7)

Corrected format string to support pyhton 2.6 (< 2.7). Current implementation only
supports python 2.7+.

>>> '{0}, {1}, {2}'.format('a', 'b', 'c')
'a, b, c'
>>> '{}, {}, {}'.format('a', 'b', 'c')  # 2.7+ only
'a, b, c'