Bump gcc version for POWER9 build

.travis.yml

@@ -35,12 +35,17 @@ matrix:
 
     - <<: *test-ubuntu
       os: linux-ppc64le
+      addons:
+        apt:
+          packages:
+           - gcc-9
+           - gfortran-9
       before_script:
         - COMMON_FLAGS="TARGET=POWER9 NUM_THREADS=32"
       env:
         # for matrix annotation only
         - TARGET_BOX=PPC64LE_LINUX_P9
-        - BTYPE="BINARY=64 USE_OPENMP=1"
+        - BTYPE="BINARY=64 USE_OPENMP=1 CC=gcc-9 FC=gfortran-9"
 
 
     - <<: *test-ubuntu