change travis gcc and gxx setting

.travis.yml

@@ -15,7 +15,7 @@ matrix:
     - os: linux
     # Compiler is specified in ./travis/common.sh
       sudo: false
-      compiler: g++-8
+      compiler: g++-6
       addons:
         apt:
           sources:
@@ -33,8 +33,8 @@ matrix:
           - doxygen
           - flex
           - fontconfig
-          - g++-8
-          - gcc-8
+          - g++-6
+          - gcc-6
           - g++-4.9
           - gcc-4.9
           - gdb

.travis/common.sh

@@ -37,7 +37,7 @@ if [[ $TRAVIS_OS_NAME == 'osx' ]]; then
   sudo installer -pkg /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg -target /
 else 
   # For linux, we use g++-8 and gcc-8 as default compilers
-  export CC=gcc-8
-  export CXX=g++-8
+  export CC=gcc-6
+  export CXX=g++-6
 fi