Remove default for local & remote in task

[mattgoldspink]

I'm using grunt-autoshot on a project to generate a simple splash for an ios Phonegap app and by default it will also try take a screenshot of google which fails when I'm offline or hangs when I'm on a slow network. I think it would be better to not set default values in the task for remote and local, it seems like you have a check later in the code to make sure one or the other set anyway.