fix travis error for MacOS

.travis.yml

@@ -66,7 +66,7 @@ matrix:
     - os: osx
       osx_image: xcode10.2 # we target latest MacOS Mojave
       sudo: true
-      compiler: gcc-4.9 # Use clang instead of gcc in MacOS
+      compiler: gcc-6 # Use clang instead of gcc in MacOS
       addons:
         homebrew:
           packages:

.travis/common.sh

@@ -31,12 +31,12 @@ function end_section() {
 
 # For Mac OS, we use g++ and gcc as default compilers
 if [[ $TRAVIS_OS_NAME == 'osx' ]]; then
-  export CC=gcc-4.9
-  export CXX=g++-4.9
+  export CC=gcc-6
+  export CXX=g++-6
   # export PATH="/usr/local/opt/bison/bin:/usr/local/bin:$PATH"
   # export PATH="/usr/local/opt/qt/bin:$PATH"
   # Install header files in Mojave, if not gcc-4.9 cannot spot stdio.h
-  sudo installer -pkg /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg -target /
+  sudo installer -pkg /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg -target / -allowUntrusted
 else
   # For linux, we use g++-8 and gcc-8 as default compilers
   export CC=gcc-8