change default compiler to gcc-6

.travis.yml

@@ -23,6 +23,8 @@ script:
   - export TRAVIS_COMMIT_LOG=`git log --format=fuller -1`
   - OUTDIR=$BASE_OUTDIR/$TRAVIS_PULL_REQUEST/$TRAVIS_JOB_NUMBER-$HOST
   - if [ -z "$NO_DEPENDS" ]; then $DOCKER_EXEC ccache --max-size=$CCACHE_SIZE; fi
+  - $DOCKER_EXEC bash -c "update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-6 60 --slave /usr/bin/g++ g++ /usr/bin/g++-6"
+  - $DOCKER_EXEC bash -c "gcc -v; g++ -v"
   - $DOCKER_EXEC bash -c "mkdir build && cd build && cmake .. && make $MAKEJOBS"
 after_script:
   - echo $TRAVIS_COMMIT_RANGE