travis36minver.txt needs an update

Travis Python 3.6 job log:
```
ERROR: packaging 19.2 has requirement pyparsing>=2.0.2, but you'll have pyparsing 2.0.1 which is incompatible.
ERROR: pandas 0.25.3 has requirement python-dateutil>=2.6.1, but you'll have python-dateutil 2.1 which is incompatible.
```
Those errors do not fail the installation (why?) and seems to had been unnoticed.