Start using GitHub actions for Linux builds

For now we switched only some of the builds, those that do not
need manually installed compiler packages to function.

.github/workflows/linux-builds.yml

@@ -0,0 +1,48 @@
+name: Linux builds
+
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-20.04
+    strategy:
+      matrix:
+        cxx: 
+#          - g++-6
+          - g++-7
+          - g++-8
+          - g++-9
+          - g++-10
+#          - clang++-7
+          - clang++-8
+          - clang++-9
+#          - clang++-10
+        build_type: [Debug, Release]
+        std: [14]
+
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Prepare environment
+      run: sudo apt install ninja-build
+
+    - name: Configure build
+      working-directory: ${{runner.workspace}}
+      env:
+        CXX: ${{matrix.cxx}}
+        CXXFLAGS: ${{matrix.cxxflags}}
+      # Note: $GITHUB_WORKSPACE is distinct from ${{runner.workspace}}.
+      #       This is important
+      run: |
+        cmake -Bbuild -H$GITHUB_WORKSPACE -DCMAKE_BUILD_TYPE=${{matrix.build_type}} \
+              -DCMAKE_CXX_STANDARD=${{matrix.std}} -DCATCH_DEVELOPMENT_BUILD=ON \
+              -G Ninja
+
+    - name: Build tests + lib
+      working-directory: ${{runner.workspace}}/build
+      run: ninja
+
+    - name: Run tests
+      working-directory: ${{runner.workspace}}/build
+      # Hardcode 2 cores we know are there
+      run: ctest -C ${{matrix.build_type}} -j 2

.travis.yml

@@ -127,14 +127,6 @@ matrix:
           packages: ['g++-7', 'lcov']
       env: COMPILER='g++-7' CPP14=1 EXAMPLES=1 COVERAGE=1 EXTRAS=1
 
-    - os: linux
-      compiler: gcc
-      addons:
-        apt:
-          sources: *all_sources
-          packages: ['g++-8']
-      env: COMPILER='g++-8' CPP17=1
-
     # OSX Clang Builds
     - os: osx
       osx_image: xcode9.4