Update base.gazebo.xacro

Update wheel contact spring values 

See mas-group/mas_industrial_robotics#5 for discussion.
These values were chosen after lots of trial and error and then finding this discussion: 
https://bitbucket.org/osrf/gazebo/issue/680/feet-drift-when-standing-due-commit

Any kp over 150000 works.

This Closes mas-group/mas_industrial_robotics#5