Add GitHub Actions workflow for CI (#6)

.github/workflows/build.yml

@@ -0,0 +1,17 @@
+name: build
+on: [pull_request]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+    - name: Set up Python 3.7
+      uses: actions/setup-python@v1
+      with:
+        python-version: 3.7
+    - name: Install dependencies
+      run: pip install -r requirements.txt
+    - name: Run syntax check
+      run: python check.py
+    - name: Build site
+      run: python build.py

.travis.yml

@@ -1,8 +0,0 @@
-language: python
-python:
-  # Current production server runs Debian stretch, which has Python 3.5.
-  - '3.5'
-install:
-  - pip install -r requirements.txt
-script:
-  - python check.py && python build.py