Add build on CentOS 7 to Travis

We recently discussed to whether is was possible for Travis to provide other build platforms like CentOS and Fedora. Travis itself doesn't provide CentOS or Fedora images, but it's possible to use Docker in a Travis build. I don't know how familiar you are with Docker, but it's a tool that provides visualization using containers. It can run a variety of images, including vanilla CentOS (version 5 to 7) and Fedora (version 20 to 24). I'm not sure if it's possible to have multiple architectures as well.

This pull request adds, in addition to the normal build that is still in place, a second build on a CentOS 7 image. It should be easy to add additional images and version, but this will increase the Travis build time.

While getting this to work, I removed the -lstdc++ flag in some makefiles because it was causing errors at some point. The change log mentioned that they should be removed and the build didn't seem affected, so I hope that this doesn't cause issues.