Migrate to GitHub Actions

[thomashoneyman]

This PR migrates lists to use GitHub Actions instead of Travis for CI, as discussed in semirings#17. It also updates the installation instructions to refer to Spago instead of Bower.

[thomashoneyman]

I've removed the MonadZero instances in this library as the class was deprecated as discussed in control#62. However, I'm not totally clear on whether we are:

• Preserving MonadZero and MonadZero instances across core until the class is removed, where control and any other library using the class will have a compiler warning for the deprecation, or
• Preserving MonadZero only in control, and removing it from the other libraries in core.

Either way, we won't be able to use the censor-lib --strict command with psa in CI anymore for at least any library which is using the class, because we want this warning to be included in the passing build. See this failed build for what happens in CI right now with the warning:

https://github.com/purescript/purescript-lists/runs/1490231947

[thomashoneyman]

I've re-added the instances to comply with the discussion in purescript/purescript-control#62. We'll have to take an additional step to get CI working with this warning.

[thomashoneyman]

Backlinking to purescript/purescript#3962

[thomashoneyman]

CI passes due to censoring user-defined warnings. Backlinking to #176.

[hdgarrood]

Thanks!