replace Travis with GitHub Actions CI (#24)

* add GitHub Actions CI
* remove Travis

.github/workflows/nodejs.yml

@@ -0,0 +1,24 @@
+name: Node CI
+
+on: [push]
+
+jobs:
+  test:
+
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        node-version: [8.x, 10.x, 12.x]
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: Use Node.js ${{ matrix.node-version }}
+      uses: actions/setup-node@v1
+      with:
+        node-version: ${{ matrix.node-version }}
+    - name: npm install, build, and test
+      run: |
+        npm install
+        npm run build
+        npm test

.travis.yml

@@ -1,4 +0,0 @@
-language: node_js
-node_js:
-  - "node"
-  - "6"