PkgBenchmark.jl workflow

I tried mimicking your .travis.yml (and used your awesome pretty printing) for Convex.jl (PR: Convex#321; travis file: https://github.com/JuliaOpt/Convex.jl/blob/master/.travis.yml). I think I messed something up though, because CI didn’t run on the next PR I made (Convex#322). Could I get some assistance? :slight_smile: