Switching to Plots.jl and unbreaking the example tests

[ali-ramadhan]

Currently the example tests are "broken", i.e. not run because having PyPlot as a dependency caused the Travis CI pipeline to frequently fail for different reasons.

I still really like matplotlib and PyPlot but after playing around with Plots.jl I think it'll be worth switching over. As it doesn't get installed through conda it should be more stable on Travis.

Would be really cool if Documenter.jl works with Plots.jl animations.

I'll give it a try.