Fix OSX build (gcc9 already installed now, so upgrade to gcc10)

Author: martin-frbg

.travis.yml

@@ -26,7 +26,7 @@ matrix:
     env: CMAKE_BUILD_TYPE=Coverage
     before_install:
     - brew update > /dev/null
-    - brew install gcc > /dev/null
+    - brew upgrade gcc > /dev/null
 
 script:
   - export PR=https://api.github.com/repos/$TRAVIS_REPO_SLUG/pulls/$TRAVIS_PULL_REQUEST