CICD: Fix the travis deployment about plotdigitizer

[dilawar]

Thanks for reporting. The Travis CI was broken for a year after they made changes to their YAML parser.

[dilawar]

• Fixed Travis and also bumped the version after removing deprecation warnings.
• Formatted source code using black.

The CI issue summarised well here https://benlimmer.com/2019/01/14/travis-ci-xvfb/ and official instructions are here https://docs.travis-ci.com/user/gui-and-headless-browsers/#using-xvfb-to-run-tests-that-require-a-gui .

Unfortunately, I could fix is satisfactory. So instead --plot showing up the plot, it takes an additional filename and saves the plot to this file.

[KikeM]

Great, I am glad your pipeline is up again ;)