v1.6.0 prep

Improvements:
* support Django 1.5 and 1.6
* remove DeprecationWarnings
* update TravisCI Django versions
* don't pin the MPTT version (0.5.5 is the last version supporting Django 1.3; there's nothing we need in 0.6.0 but we shouldn't hold people back)
* Readme updates

New Features:
* provide reasonable default Section ordering