Switch to GitHub actions for testing (#24)

Travis is a bit flaky, see flycheck/emacs-travis#22.

.github/workflows/main.yml

@@ -0,0 +1,29 @@
+name: CI
+
+on: [pull_request, push]    
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        emacs_version:
+          - 24.3
+          - 24.4
+          - 24.5
+          - 25.1
+          - 25.2
+          - 25.3
+          - 26.1
+          - 26.2
+          - 26.3
+          - snapshot
+    steps:
+    - uses: purcell/setup-emacs@master
+      with:
+       version: ${{ matrix.emacs_version }}
+    - name: Install cask
+      run: git clone --depth=1 https://github.com/cask/cask.git "${HOME}/.cask" && sudo ln -s "${HOME}/.cask/bin/cask" "/usr/local/bin/cask"
+    - uses: actions/checkout@v1
+    - name: Run tests
+      run: make setup test