Run CI on GitHub Actions, not Travis

* Run CI on GitHub Actions, not Travis - refs #940
* Update documentation refs to Travis
* Release action now runs parallel tests, then pushes to PyPI, then Docker Hub

.github/workflows/test.yml

@@ -0,0 +1,29 @@
+name: Test
+
+on: [push]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: [3.6, 3.7, 3.8]
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Python ${{ matrix.python-version }}
+      uses: actions/setup-python@v2
+      with:
+        python-version: ${{ matrix.python-version }}
+    - uses: actions/cache@v2
+      name: Configure pip caching
+      with:
+        path: ~/.cache/pip
+        key: ${{ runner.os }}-pip-${{ hashFiles('**/setup.py') }}
+        restore-keys: |
+          ${{ runner.os }}-pip-
+    - name: Install dependencies
+      run: |
+        pip install -e '.[test]'
+    - name: Run tests
+      run: |
+        pytest