give up an xenial too many overrides, bionic it is

.travis.yml

@@ -24,14 +24,14 @@ env:
 install:
     - env | grep -E '^(CCACHE_|DISPLAY|CONFIG_SHELL)' | tee /tmp/env
     - if [[ $HOST = *-mingw32 ]]; then DOCKER_ADMIN="--cap-add SYS_ADMIN"; fi
-    - DOCKER_ID=$(docker run $DOCKER_ADMIN -idt --mount type=bind,src=$TRAVIS_BUILD_DIR,dst=$TRAVIS_BUILD_DIR --mount type=bind,src=$CCACHE_DIR,dst=$CCACHE_DIR -w $TRAVIS_BUILD_DIR --env-file /tmp/env ubuntu:16.04)
+    - DOCKER_ID=$(docker run $DOCKER_ADMIN -idt --mount type=bind,src=$TRAVIS_BUILD_DIR,dst=$TRAVIS_BUILD_DIR --mount type=bind,src=$CCACHE_DIR,dst=$CCACHE_DIR -w $TRAVIS_BUILD_DIR --env-file /tmp/env ubuntu:18.04)
     - DOCKER_EXEC="docker exec $DOCKER_ID"
     - if [ -n "$DPKG_ADD_ARCH" ]; then $DOCKER_EXEC dpkg --add-architecture "$DPKG_ADD_ARCH" ; fi
     - travis_retry $DOCKER_EXEC apt-get update
     - travis_retry $DOCKER_EXEC apt-get install --no-install-recommends --no-upgrade software-properties-common -qq $PACKAGES $DOCKER_PACKAGES
-    - travis_retry $DOCKER_EXEC add-apt-repository ppa:ubuntu-toolchain-r/test
-    - travis_retry $DOCKER_EXEC apt-get update
-    - travis_retry $DOCKER_EXEC apt-get install --no-install-recommends --no-upgrade -qq gcc-6 g++-6
+#    - travis_retry $DOCKER_EXEC add-apt-repository ppa:ubuntu-toolchain-r/test
+#    - travis_retry $DOCKER_EXEC apt-get update
+#    - travis_retry $DOCKER_EXEC apt-get install --no-install-recommends --no-upgrade -qq gcc-6 g++-6
 script:
   - export TRAVIS_COMMIT_LOG=`git log --format=fuller -1`
   - OUTDIR=$BASE_OUTDIR/$TRAVIS_PULL_REQUEST/$TRAVIS_JOB_NUMBER-$HOST