Bump default heka version to 0.7.1

Full list of changes available [here](https://github.com/mozilla-services/heka/blob/dev/CHANGES.txt)

The main item here is the bump to version 0.7.1 from 0.6.0 as the default version.

Updates to the README made to keep in line with changes
* Tested convergence on Ubuntu 14.04
* Update default attributes listed
* Add notes on required override attributes for a different version