Version 5.4 of pytest is not supported

[HelloThisIsFlo]

In version 5.4.* of pytest, an error is thrown when capturing the logging

TerminalReporter.writer attribute is deprecated, use TerminalReporter._tw instead at your own risk.
  See https://docs.pytest.org/en/latest/deprecations.html#terminalreporter-writer for more information.

I have no time to investigate it now, so I pinned down the version 5.3.* so that travis build stop failing but this is obviously an issue we will want to tackle 🙂
Quick fix commit: 5873630f0979381bdec751729cf648037fbb681c

I'll look at it in the following weeks 🙂