Changes VisTrails URL to Github

As indicated by my email on Oct 15, I think UV-CDAT should really be using a Github fork of VisTrails to host the VisTrails repository and all the branches. That would allow us developers to use pull requests the way we do for changes on the main UV-CDAT repository (this one, UV-CDAT/uvcdat) and uvcdat-testdata, instead of pushing to vistrails.org and commenting with a branch name.

I am unaware of further discussion on this (didn't see anything on Github or the mailing-list), feel free to comment.

A nightly script updates the fork from vistrails.org right now, we can make it do the contrary when you decide to switch.