update Travis-CI to use newer version of gcc

.travis.yml

-language: c++
+language: cpp
 compiler: gcc
 dist: trusty
 
+before_install:
+  - sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y
+  - sudo apt-get update -q
+  - sudo apt-get install g++-6 -y
+  - sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-6 90
+
 addons:
   apt:
     packages: