Setup Travis

[marfox]

It should run the following commands:

isort -y soweego # organize imports
autoflake -r --remove-all-unused-imports --remove-unused-variables --in-place soweego # remove unused imports and variables
black -l 80 soweego # split lines longer than 80 chars
flake8 soweego # linter report

[marfox]

• We opted not to enable the linter, because it almost certainly fails the build for potentially trivial details;
• we let Travis push back formatted code in the travis branch, inspired by https://stackoverflow.com/a/33125422/10719765 and https://gist.github.com/MichaelCurrie/802ce28c993ff2dd632c