fix gfortran requirement in osx interface64 test

.travis.yml

@@ -216,8 +216,10 @@ matrix:
         - COMMON_FLAGS="DYNAMIC_ARCH=1 NUM_THREADS=32"
       script:
         - travis_wait 45 make QUIET_MAKE=1 $COMMON_FLAGS $BTYPE
+        - brew update
+        - brew install gcc-10
       env:
-        - BTYPE="TARGET=NEHALEM BINARY=64 INTERFACE64=1 FC=gfortran-8"
+        - BTYPE="TARGET=NEHALEM BINARY=64 INTERFACE64=1 FC=gfortran-10"
 
     - <<: *test-macos
       osx_image: xcode12