Fix Travis CI requirements to use django-mptt==0.6.0

django-mptt 0.6.1 is broken, thus force version to 0.6.0 in Travis
CI requirements too. This had been done in normal requirements.txt
already.
Merge pull request #317 from jluttine/fix-travis-mptt

Fix Travis CI requirements to use django-mptt==0.6.0
1. Add template variables
2. Add template plugins README.md
Merge commit '55eb10aa7671c61e038c9dc650a1f26137d1fe6c' into plugin_template