[travis] fix package sources

.travis.yml

@@ -4,11 +4,13 @@ sudo: false
 
 addons:
   apt:
+    sources:
+      - ubuntu-toolchain-r-test
     packages:
       - gdb
       - python-dbg
       - python3-dbg
-      - gcc-5
+      - gcc-6
 
 cache:
   pip: true
@@ -89,8 +91,8 @@ before_install:
     if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then
       mkdir "$HOME/gcc-symlinks"
       export PATH="$HOME/gcc-symlinks:$PATH"
-      ln -s $HOME/gcc-symlinks/gcc /usr/bin/gcc-5
-      ln -s $HOME/gcc-symlinks/g++ /usr/bin/gcc-5
+      ln -s $HOME/gcc-symlinks/gcc /usr/bin/gcc-6
+      ln -s $HOME/gcc-symlinks/g++ /usr/bin/gcc-6
 
       which gcc
       which g++