travis: Use GCC 6 on Ubuntu CI builds

.travis-build.sh

@@ -11,8 +11,8 @@ fi
 
 #if OS is linux or is not set
 if [ "$TRAVIS_OS_NAME" = "linux" -o -z "$TRAVIS_OS_NAME" ]; then
-    export CC=gcc-5
-    export CXX=g++-5
+    export CC=gcc-6
+    export CXX=g++-6
     export PKG_CONFIG_PATH=$HOME/.local/lib/pkgconfig:$PKG_CONFIG_PATH
 
     mkdir build && cd build

.travis-deps.sh

@@ -5,8 +5,8 @@ set -x
 
 #if OS is linux or is not set
 if [ "$TRAVIS_OS_NAME" = "linux" -o -z "$TRAVIS_OS_NAME" ]; then
-    export CC=gcc-5
-    export CXX=g++-5
+    export CC=gcc-6
+    export CXX=g++-6
     mkdir -p $HOME/.local
 
     curl -L http://www.cmake.org/files/v3.1/cmake-3.1.0-Linux-i386.tar.gz \

.travis.yml

@@ -17,8 +17,8 @@ addons:
     sources:
       - ubuntu-toolchain-r-test
     packages:
-      - gcc-5
-      - g++-5
+      - gcc-6
+      - g++-6
       - qt5-default
       - libqt5opengl5-dev
       - xorg-dev