Re-enable Coveralls support

[alexcanepa]

I haven't been here that long but it looks like the repo used to do coverage testing when it was hosted under @simonster: https://coveralls.io/github/simonster/MAT.jl

It seems like this was last run in May 2017. It would be great to try and get this running again, but I'm assuming an org member would need to set it up.

As a side note, the after_success section in Travis needs to be updated from:

julia -e 'cd(Pkg.dir("MAT")); Pkg.add("Coverage");
using Coverage; Coveralls.submit(Coveralls.process_folder())'

to:

julia -e 'using Pkg; import MAT; cd(joinpath(dirname(pathof(MAT)), ".."));
Pkg.add("Coverage"); using Coverage; Coveralls.submit(Coveralls.process_folder())'

The current version throws an error without "using Pkg". The additional changes follow from a deprecation warning for Pkg.dir() in Julia 1.1.