Added velocity damping to pusher, fixes #477

[AwesomeLemon]

As described in https://github.com/google/brax/issues/477, currently there is no friction-like force in the pusher environment with the spring backend. This means that even a slight touch of the object by the robot leads to the object infinitely sliding off into the distance.

Per suggestion of @btaba in that issue, I added velocity damping, which solved the problem.

Note that I had to set its value to -1, despite it being set to either 0 or 1 in the Brax codebase. Looking at the integrator.py code shows that having vel_damping > 0 leads to anti-damping, as xd_i.vel is multiplied by exp(positive number) > 1.