Update ECOS.jl information

@jfsantos

The ECOS interface started by @jfsantos has actually been completed by myself and integrated into the Julia optimization interface. It also has support for automatically downloading and building on Linux and OSX, and we're about to add binary download support for Windows. Hopefully it'll be one of the easiest ways for people to download and try ECOS.

This PR updates the link away from a fork and points to the true, final location, with [appropriate credits](https://github.com/JuliaOpt/ECOS.jl/graphs/contributors)