Push out v2.6 to gather up local fixes and updates
PR #244 Polished README.rst
PR #242 fix pyicu import to suppress warnings
PR #239 Fixed missing comma in seconds strings
Updated Pipfile and Makefile to:
- update and move packages to the "dev" section
- use Python 3.7 for pipenv
- install tox-pipenv plugin to try and fix Tox (currently doesn't)
- simplify tox.ini to try and fix Tox (didn't)
- move ci makefile target to the circle config