Use gcc-5 to build on Travis CI
Use gcc-5 to build on Travis CI

I found this solution on this [blog](https://genbattle.bitbucket.io/blog/2016/01/17/c++-travis-ci/?).
We now use C++11.

mesnardo/PetIBM,barbagroup/PetIBM,mesnardo/PetIBM,mesnardo/PetIBM,piyueh/PetIBM