title: Building with more than one version of a compiler
body: <p>How can I configure travis-ci such that my project builds with more than one version of a compiler?</p>
  
  <p>Say, I want to build it with gcc-4.8, gcc-4.9, clang-3.4, clang-3.5 and clang-3.6.</p>
  
  <p>I know how to build on both gcc and clang, but not on more than one version of them.</p>
  
  <p>To give a little bit more context, my <a href="https://github.com/facebook/fatal/tree/dev#fatal-facebook-template-library-">project</a> is a C++ library and I want to ensure compatibility with those compilers.</p>