[PlayerStage-Gazebo] Terrain Model fix

[Nate K. <nat...@gm...>]

Hello,

The last release of Gazebo has a problem with the Terrain Model.
Objects can fall through it at certain points.

This has been fixed. Since no one has complained, I guess it is not
that big of a deal. But if you need it, get the latest CVS version and
all will be well.

-nate