Add GitHub Actions CI builds

[jidicula]

Checklist

• I've given this PR a concise, self-descriptive, and meaningful title
• I've linked relevant issues in the PR body
• I've applied the relevant labels to this PR
• I've added relevant tests for my contribution
• I've updated the documentation and/or added correct docstrings
• I've assigned a reviewer

Description

Why this change was necessary
Travis doesn't offer great support for macOS and is also beginning to
charge for macOS build minutes.

What this change does

• Refactors the Travis CI build config into a shell script for unit tests
• Calls that shell script in a GitHub Actions Workflow after setting up dependencies
• Adds a matrix build in the GH Actions Workflow for macOS 10.15
  Catalina and 11.0 Big Sur (on x86_64 Intel chips), and Ubuntu 18, 20, and
  16
• Adds a matrix build in the GH Actions Workflow for Python 3.7 and
  3.8
• Adds a build status badge to the README

Additional context/notes/links
https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing

Linked issues

Resolves #180

[jidicula]

@jcohenadad I think it's good to go now. All the GH Actions runs are passing (love those green checkmarks 😁) and the Python version build matrix looks fine too!

Note that I've set up Coveralls to only run on the Ubuntu 18.04 build. The tiny decrease in coverage looks like it's only on Python 3.8 (did some detective-work with which build finished first and matched it with the reports on coveralls.io). There might be a way to do some kind of averaging over all the build matrix OS options but I need to look into it further – I think it's best to handle that in a different PR.

The Coveralls builds run over all matrix builds now and I believe it averages coverage over each build parameter. 3.8 seems to have fewer lines run under the hood, giving it the appearance of having lower coverage. However, since the build matrix will be running in every PR CI check from now on, it's really just setting a new baseline coverage percentage that future PRs will be measured against.

[po09i]

I looked more closely at the builds and even though they say they pass, they all have errors within the CI run. I think its related to prelude and dcm2niix not being installed.

[jidicula]

I looked more closely at the builds and even though they say they pass, they all have errors within the CI run. I think its related to prelude and dcm2niix not being installed.

Oh good catch - maybe a ci.sh isn't such a great idea since it obscures this stuff... I'll try to fix this up

[jidicula]

@po09i @jcohenadad I narrowed down the failing build to the macOS build needing the macOS prelude binary. I would like to add the macOS prelude binary to the binaries repo, then conditionally set up some steps in the GH Actions workflow to use the correct binary depending on what build matrix OS option is running. Could you give me push access to the binaries repo?

[po09i]

LGTM!