[ros-users] edits to gentoo install wiki page

Some quick notes on the install page the gentoo on the wiki:

add
> echo "dev-python/setuptools" >> /etc/portage/package.keywords

and:
> emerge -u setuptools

in order to get a version of setuptools new enough so that
'easy_install rosinstall' will work.

Also, change line 55 of $ROS_ROOT/rosdep.yaml from:
gentoo: "=dev-python/pygtk-2.*"
to:
gentoo: "=dev-python/pygtk-2*"

in order to make it a valid package atom.

-Dave