Run EPUBCheck on CI

This should help detect any issues such as those seen in #22.

.github/workflows/test.yml

@@ -0,0 +1,50 @@
+name: Test
+on:
+  pull_request:
+    branches:
+    - master
+  push:
+    branches:
+    - master
+jobs:
+  test:
+    name: Test
+    runs-on: ubuntu-20.04
+    steps:
+    - uses: actions/checkout@v2
+
+    - uses: actions/setup-node@v1
+      with:
+        node-version: 14
+
+    - run: npm install
+
+    - run: npm run lint
+
+    # CI would take too long if we did the download every time too. So, we cache it. This does mean we're vulnerable to
+    # source changes exposing problems in our code, but those are pretty infrequent. If they occur, we need to bump the
+    # cache key.
+    - uses: actions/cache@v2
+      with:
+        key: worm-ward-cache-2020-12-31
+        path: ./cache
+
+    - run: node ./lib/worm-scraper.js --book=worm
+
+    - run: node ./lib/worm-scraper.js --book=ward
+
+    - uses: actions/setup-java@v1
+      with:
+        java-version: 15
+        java-package: jre
+
+    - name: Get EPUBCheck
+      run: |
+        curl https://github.com/w3c/epubcheck/releases/download/v4.2.4/epubcheck-4.2.4.zip --location --output epubcheck.zip
+        unzip epubcheck.zip
+
+    - name: Check Worm.epub
+      run: java -jar epubcheck-4.2.4/epubcheck.jar --failonwarnings Worm.epub
+
+    - name: Check Ward.epub
+      run: java -jar epubcheck-4.2.4/epubcheck.jar --failonwarnings Ward.epub

.travis.yml

@@ -1,6 +0,0 @@
-language: node_js
-node_js:
-  - 10
-  - stable
-script:
-  npm run lint