Use Github Actions as CI

[jjlawren]

Travis has recently introduced a new pricing model which has impacted the CI runs for free plans. I've observed CI runs for this repo queued for over an hour before executing.

This PR provides the necessary config to use Github Actions as our CI instead. An example run of this config on my fork is available here: https://github.com/jjlawren/python-plexapi/actions/runs/369388047.

A few outstanding items that need to be addressed:

• Set repo env secrets for PLEXAPI_AUTH_MYPLEX_USERNAME and PLEXAPI_AUTH_MYPLEX_PASSWORD (@pkkid)
• Add teardown step to remove temporary CI PMS/client from myplex account
• Add coveralls.io (or codecov.io) integration
• Add an action to publish to PyPI (currently handled by Travis) (Github action to publish to PyPI #443)
• Remove Travis configuration

[Hellowlol]

@pkkid can you setup the secrets in the repo?

[jjlawren]

Not sure how much more I can validate these runs on my own repo. Tests are running reliably and now uploading coverage reports to codecov.io: https://github.com/jjlawren/python-plexapi/actions/runs/395247820.

[Hellowlol]

Great work! Secrets etc has been checked, is this something you have done?

[JonnyWong16]

TODO: Update badges in readme.

[JonnyWong16]

Adds caching of the plexinc/pms-docker image so that it does not need to be pulled repeatedly from DockerHub.

See Docker cache workflow run here:
https://github.com/JonnyWong16/python-plexapi/runs/1490367581

[jjlawren]

I think this is finally in a good place and is ready to merge. Lots of thanks to @JonnyWong16 for many clever suggestions and additional validation.

Badge on the repo has been changed to codecov.io. Travis is still in place, but can be decommissioned once we're comfortable here. Once Travis is disabled, #443 should be updated and merged.

[Hellowlol]

Lets go for it