Improve Travis

[iff]

• use matrix for clang/gcc openmpi/mpich
• make use of the Elemental deb (once it becomes available)

[andreasnoack]

I have a matrix running in https://github.com/JuliaParallel/Elemental.jl/blob/master/.travis.yml. Unfortunately, the ccache part is not working as well as it should but I still think it gives at significant speedup.