Replace Travis CI with GitHub actions

.github/workflows/linux.yml

@@ -0,0 +1,42 @@
+name: Linux
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+
+concurrency:
+  group: ${{ github.ref }}-${{ github.workflow }}-${{ github.event_name }}
+  cancel-in-progress: true
+
+jobs:
+  build:
+    name: build
+    runs-on: ubuntu-20.04
+    continue-on-error: true
+
+    strategy:
+      matrix:
+        compiler:
+          - { compiler: GNU,  CC: gcc,   CXX: g++,    }
+          - { compiler: LLVM, CC: clang, CXX: clang++ }
+
+    steps:
+      - name: checkout
+        uses: actions/checkout@v2
+
+      - name: cmake
+        env:
+          CC: ${{ matrix.compiler.CC }}
+          CXX: ${{ matrix.compiler.CXX }}
+        run: |
+          cmake -B ${{ github.workspace }}/build
+
+      - name: build library
+        run: |
+          cmake --build ${{ github.workspace }}/build
+
+      - name: run tests
+        working-directory: ${{ github.workspace }}/build
+        run: |
+          ctest