Replace web repl picture

[ViralBShah]

We should replace the web repl picture on the front page with one based on IJulia

Cc: @stevengj @JeffBezanson @StefanKarpinski

[stevengj]

Sounds good, but probably best to wait a few days until the packaging of IJulia becomes a bit nicer.

Also, does this depend on JuliaLang/IJulia.jl#1?

[ViralBShah]

I agree that we should wait for IJulia to become easier to install. I don't think it should depend on JuliaLang/IJulia.jl#1 although I will keep pushing on it.

[StefanKarpinski]

Did this.