This should be the proper way.

.travis.yml

@@ -1,41 +1,34 @@
 dist: bionic
 language: c
-os:
-   - linux
-   - osx
 
 addons:
   apt:
     packages:
       - lcov
 
+os:
+  - linux
+  - osx
 compiler:
-   - clang
+  - clang
-   - gcc
+  - gcc
+
+script:
+  - mkdir build
+  - cd build
+  - cmake ..
+  - make test
 
 jobs:
-   include:
+  include:
-      - stage: "Build"
+    - stage: "Coverage"
-        script:
+      os: linux
-          - mkdir build
+      compiler: clang
-          - cd build
+      script:
-          - cmake ..
+        - mkdir coverage
-          - make
+        - cd coverage
-          - cd ..
+        - cmake .. -DCOVERALLS=ON -DCMAKE_BUILD_TYPE=Debug
-      - stage: "Tests"
+        - cmake --build .
-        name: "Unit tests"
+        - cmake --build . --target coveralls
-        script:
+      after_success:
-          - cd build
+        - bash <(curl -s https://codecov.io/bash)
-          - make test
-          - cd ..
-      - stage: "Coverage"
-        name: "Running coverall coverage"
-        script:
-          - mkdir coverage
-          - cd coverage
-          - cmake .. -DCOVERALLS=ON -DCMAKE_BUILD_TYPE=Debug
-          - cmake --build .
-          - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then cmake --build . --target coveralls; fi
- 
-after_success:
-   - bash <(curl -s https://codecov.io/bash)