Move to Github Actions

[Matt Trask]

Now that Github Actions are gaining wide usage, we should drop Travis and Circle and solely use Github actions. Less service integration to maintain.

[Korvin Szanto]

I'd love to see this working, we might be able to get the change back into https://github.com/thephpleague/skeleton if we can make it generic 👍 👍

[Steve McDougall]

@matthewtrask want me to handle this one?

[Matt Trask]

@JustSteveKing that would be great!

[Korvin Szanto]

Thanks @JustSteveKing, next steps on this are:

1. Move the adapter / paginator repos over to github actions as well
2. Update this repo to only test 5.6+

[Steve McDougall]

No problem @KorvinSzanto if I'm honest I'd be tempted to raise the min PHP version to the officially supported one.

I'm happy to look at the pagination adapters repo if needed, just give me a shout!