Note the requirements for C++14 in CONTRIBUTING.md, update travis build

scripts/travis/linux_compile.sh

@@ -7,6 +7,12 @@ set +e
 
 set -e
 
+# Switch to the gcc version we want
+if [ $CC == "gcc" ]; then
+	export CC=gcc-6;
+	export CXX=g++-6;
+fi
+
 mkdir build
 cd build
 cmake -DCMAKE_BUILD_TYPE=Debug ..

scripts/travis/linux_setup.sh

@@ -6,7 +6,7 @@ sudo add-apt-repository -y 'ppa:beineri/opt-qt562-trusty'
 sudo add-apt-repository -y 'deb http://apt.llvm.org/precise/ llvm-toolchain-precise-3.8 main'
 wget -O - http://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
 sudo apt-get update -qq 
-sudo apt-get install --allow-unauthenticated -y -qq libx11-dev mesa-common-dev libgl1-mesa-dev qt56base qt56x11extras libxcb-keysyms1-dev gdb clang-format-3.8
+sudo apt-get install --allow-unauthenticated -y -qq libx11-dev mesa-common-dev libgl1-mesa-dev qt56base qt56x11extras libxcb-keysyms1-dev gdb clang-format-3.8 g++-6
 
 # check formatting matches clang-format-3.8. Since newer versions can have
 # changes in formatting even without any rule changes, we have to fix on a