Switch to gcc5, provide stdlib for clang builds

Trying to fix CI

.travis.yml

@@ -4,7 +4,7 @@ language: cpp
 install:
   - sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
   - sudo apt-get -qq update
-  - sudo apt-get install -y g++-4.8 libstdc++-4.8-dev
+  - sudo apt-get install -y g++-5 libstdc++-5-dev
   - sudo apt-get install -y flex valgrind
   - sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-4.8 90
   - sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.8 90
@@ -22,6 +22,7 @@ install:
   - bison --version
   - flex --version
   - valgrind --version
+  - if [ "$CXX" = "clang++" ]; then export CXXFLAGS="-stdlib=libc++"; fi
 
 compiler:
   - gcc