Replace defunct Travis CI integration with GitHub Actions

The GitHub Actions setup is equivalent to the Travis CI configuration
with two differences:

- The GitHub Action setup does not set the `TAG` environment variable,
  as it didn't seem like `xml2rfc` would consume that at all (unless I'm
  missing something)

- The GitHub Action preserves the spec output (txt and html) as
  artifacts that can be downloaded after a successful run (to see the
  changes without having to locally render the specs)

Fixes: #1262
Signed-off-by: Juan Cruz Viotti <jv@jviotti.com>

Author: jviotti

.github/workflows/ci.yml

@@ -0,0 +1,14 @@
+name: JSON Schema
+on:
+  - push
+jobs:
+  specs:
+    runs-on: ubuntu-22.04
+    steps:
+      - uses: actions/checkout@v3
+      - run: pip install --requirement requirements.txt
+      - run: xml2rfc --version
+      - run: make all
+      - uses: actions/upload-artifact@v2
+        with:
+          path: *.(html|txt)