Autoscale Tweaks

Racks set to Autoscale=Yes will now add instances in groups but remove them one at a time. This will help the ECS scheduler rebalance without pulling the rug out from under it.

Enhancements

• #1215 update help text for --no-sync [@mattmanning]
• #1223 upgrade go-dockerclient [@ddollar]
• #1230 autoscale down slowly [@ddollar]