make attempt_version_bump class Version
test serialising only partial version
sys.stdout is utf-8 encoded (fixes #13)
make global variable a class variable
Print own version number in usage (fixes #12)
fix reading/writing utf-8 files
-confusing "feature": leave current_version cfg

Fixes #4
Fixes #17
Code beautification
load updated 3.x configparser first

.. only then try the old one.
use correct file encoding writing to config file

Fixes #18
allow utf-8 message config test to fail with py27

… only if the configparser module is not installed

also add extra test environment with updated
configparser
py27: Warn users about pip install -U configparser
travis: use the latest pytest version
use tox on travis to test second py27-env
don't execute all tox envs on all python versions