travis: use gcc 8

.travis.yml

@@ -43,23 +43,23 @@ before_install:
   - if [ ${TRAVIS_OS_NAME} = "osx" ]; then
       brew install fftw || true;
     fi
-
-install:
   - git submodule update --init --recursive
-  - if [[ ${COVERITY_BUILD_DISABLED} == 1 ]];
-    then
-        exit 0;
-    fi
   - if [ ${TRAVIS_OS_NAME} = "linux" -a "$CC" = "gcc" ];
     then
         if [ "$BUILDDOC" = 1 ]; then
             sudo apt-get install doxygen xsltproc;
         fi;
         sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test;
         sudo apt-get update;
-        sudo apt-get install gcc-7 g++-7;
-        sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-7 90;
-        sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-7 90;
+        sudo apt-get install gcc-8 g++-8;
+        sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-8 90;
+        sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-8 90;
+    fi
+
+install:
+  - if [[ ${COVERITY_BUILD_DISABLED} == 1 ]];
+    then
+        exit 0;
     fi
 
 # CImg is now downloaded by CImg/Makefile, no need to install it
@@ -85,8 +85,8 @@ addons:
     - ubuntu-toolchain-r-test
     packages:
     - ca-certificates
-    - gcc-7
-    - g++-7
+    - gcc-8
+    - g++-8
     - libfftw3-dev
   coverity_scan:
     # customized build script URL