Update .travis.yml

Author: martin-frbg

.travis.yml

@@ -227,7 +227,7 @@ matrix:
       script:
         - travis_wait 45 make QUIET_MAKE=1 $COMMON_FLAGS $BTYPE
       env:
-        - BTYPE="TARGET=HASWELL USE_OPENMP=1 BINARY=64 INTERFACE64=1 FC=gfortran-10"
+        - BTYPE="TARGET=HASWELL USE_OPENMP=1 BINARY=64 INTERFACE64=1 CC=gcc-10 FC=gfortran-10"
 
     - <<: *test-macos
       osx_image: xcode12