Add GitHub actions CI.

Author: jgm

.github/workflows/ci.yml

@@ -0,0 +1,28 @@
+name: CI tests
+
+on: [push, pull_request]
+
+jobs:
+
+  linux:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        versions:
+          - node: '13.x'
+          - node: '12.x'
+          - node: '11.x'
+          - node: '10.x'
+          - node: '9.x'
+          - node: '8.x'
+          - node: '7.x'
+    uses: actions/checkout@v1
+- name: Use Node.js
+  uses: actions/setup-node@v1
+  with:
+    node-version: ${{ matrix.versions.node }}
+    steps:
+      - run: npm install
+      - run: npm run build --if-present
+      - run: npm test