Replace Travis with GitHub actions

.github/workflows/ci.yml

@@ -0,0 +1,11 @@
+name: CI
+
+on: pull_request
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Run shellcheck
+      run: ci/run_shellcheck.sh

.travis.yml

@@ -1,8 +0,0 @@
----
-dist: bionic
-language: bash
-cache:
-  directories:
-  - "/opt/shellcheck"
-script:
-- ci/run_shellcheck