Migrate away from travis

[russelljjarvis]

NeuralEnsemble/libNeuroML#97

[russelljjarvis]

From: NeuralEnsemble/libNeuroML#97
So, it's not looking like a good long term plan to migrate to travis-ci.com. It looks like GH Actions is the preferred replacement: it also gets rid of the administration that third party CI services require. GitLab CI seems to be the ideal candidate but for those with lots of repos on GitHub migrating will require quite a bit of work---which is perhaps overkill at the moment.`

[russelljjarvis]

It might be worth copying BluePyOpt and using tox+Github Actions.

Tox can be run locally and it has a lot of python specific functionality.

https://tox.readthedocs.io/en/latest/index.html