gazebo3: disable DART support
gazebo3: disable DART support

gazebo3 only supports DART 3.x, while the latest DART
version is 4.x, so disable to prevent build failures

osrf/homebrew-simulation,hgaiser/homebrew-simulation,j-rivero/homebrew-simulation,scpeters/homebrew-simulation,NikolausDemmel/homebrew-simulation