Change vision_visp location

Although github is able to do a "symbolic link" between both
git://github.com/laas/vision_visp and git://github.com/lagadic/vision_visp,
there might be an issue if the difference between the remote repository and
the local repository is too big.
Especially if the local repository still uses the submodule (5d53b7d7),
the git pull will not pass smoothly (hence the rosinstall command will crash).

In this case, the simplest solution is to force the checkout of the new repository,
which is what this modification does.
Note that when running the rosinstall command, you'll be prompted
to confirm this change of repository  and asked if you want to 
keep your former repository (ie if you have local changes).