Fixes to maintain support for python 2.6

Two very small changes here that were causing problems in 2.6:

1. Substitution using "{} {} {}".format(a, b, c) requires explicit indices in 2.6 e.g. "{0} {1} {2}"
2. The previous check for python 2.6 in indexer.py wasn't picking up 2.6.6, and therefore was trying to run python 2.7+ code