Begins jinja2 support as mentioned in ARMmbed/yotta#90
add jinja2 to dependencies, remove Cheetah
Add Python3 to .travis.yml
Revert "Add Python3 to .travis.yml"

This reverts commit 74bf7eafdffa76be18283e4d08e210ebdaba41da.
Merge branch 'jinja2' of https://github.com/matthewelse/yotta into matthewelse-jinja2
logging level must be integer in python 3