Minimum Python version is now 2.6.

@matthewrmshin - please review (a trivial one)

Closes #1019. [ok, closing issues automatically by keyword only works for pull requests to master]
Can do a one liner.

```sh
if python -c 'import sys; sys.exit(sys.version_info < (2, 6))'; then
    echo ok
else
    echo "ERROR: Python version too old"
fi
```