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