Travis CI is a platform where you can test the software you build after every Github push (in my case).
After you have did the push, Github send a request to Travis and it start to clone and build your project about the instructions from

.travis.yml

configuration file from your repo.
But, this time I will tell you how to instruct Travis to compile your code with GCC 5 compiler version. And it’s very simple. I was needed to do that while I was working at WebDollar cryptocurrency project.
Bellow you have the config of Travis:

{
  "language": "node_js",
  "sudo": "required",
  "before_install": [
    "npm install -g node-gyp"
  ],
  "node_js": "stable",
  "env": "CXX=g++-5",
  "addons": {
    "apt": {
      "sources": [
        "ubuntu-toolchain-r-test"
      ],
      "packages": [
        "g++-5"
      ]
    }
  },
  "group": "stable",
  "dist": "trusty",
  "os": "linux"
}

As you can see, in the apt section, in sources I have the ubuntu-toolchain-r-test. This source contain the g++ version 5 compiler.
Bellow apt section I instruct the environment to use that GCC compiler.