Add GitHub Actions CI builds

Resolves #180

**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**
 - Factors the Travis CI build config into a shell script
 - Calls that shell script in a GitHub Actions Workflow
 - Adds a matrix build in the GH Actions Workflow for macOS 10.15
 Catalina and 11.0 Big Sur (on Apple Silicon), 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
 - Adds Coveralls coverage metrics for the Ubuntu 18.04 build only.

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

.github/workflows/tests.yml

@@ -0,0 +1,49 @@
+name: CI-Tests
+
+on:
+  # Triggers the workflow on push or pull request events but only for the master branch
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+  # Allows you to run this workflow manually from the Actions tab
+  workflow_dispatch:
+
+jobs:
+  test:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os:
+          - 'macos-10.15'
+          - 'macos-11.0'
+          - 'ubuntu-18.04'
+          - 'ubuntu-20.04'
+          - 'ubuntu-16.04'
+        python-version:
+          - '3.7'
+          - '3.8'
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Set XCode version
+        if: contains(matrix.os, 'macos-11.0')
+        run: sudo xcode-select -s "/Applications/Xcode_12.2.app"
+
+      - name: Set up Python
+        uses: actions/setup-python@v2
+        with:
+          python-version: ${{ matrix.python-version }}
+
+      - name: Run CI script
+        run: ./ci.sh
+
+      - name: Coveralls
+        if: contains(matrix.os, 'ubuntu-18.04')
+        env:
+          GITHUB_TOKEN: ${{ secrets.github_token }}
+        run: |
+          coveralls

README.md

@@ -1,6 +1,6 @@
 # shimming-toolbox
 
- [![Build Status](https://github.com/shimming-toolbox/shimming-toolbox/actions?query=workflow%3A%22CI-Tests/badge.svg)](https://github.com/shimming-toolbox/shimming-toolbox/actions?query=workflow%3A%22CI-Tests) [![Build Status](https://travis-ci.com/shimming-toolbox/shimming-toolbox.svg?branch=master)](https://travis-ci.com/shimming-toolbox/shimming-toolbox) [![Documentation Status](https://readthedocs.org/projects/shimming-toolbox/badge/?version=latest)](https://www.shimming-toolbox.org/en/latest/?badge=latest) [![Coverage Status](https://coveralls.io/repos/github/shimming-toolbox/shimming-toolbox/badge.svg?branch=master)](https://coveralls.io/github/shimming-toolbox/shimming-toolbox?branch=master)
+ [![Build Status](https://github.com/shimming-toolbox/shimming-toolbox/actions?query=workflow%3A%22CI-Tests/badge.svg)](https://github.com/shimming-toolbox/shimming-toolbox/actions?query=workflow%3A%22CI-Tests) [![Documentation Status](https://readthedocs.org/projects/shimming-toolbox/badge/?version=latest)](https://www.shimming-toolbox.org/en/latest/?badge=latest) [![Coverage Status](https://coveralls.io/repos/github/shimming-toolbox/shimming-toolbox/badge.svg?branch=master)](https://coveralls.io/github/shimming-toolbox/shimming-toolbox?branch=master)
 
 
 Code for performing real-time shimming using external MRI shim coils