Do not send environment as ROLLBACK_TARGET

Release tools does not expect the environment to be sent as ROLLBACK_TARGET or ROLLBACK_CURRENT. Sending nil in both will cause release tools to check rollback with the latest 2 deployments to the given environment that is sent in ROLLBACK_ENV.

Release tools now only expects version strings in ROLLBACK_TARGET and ROLLBACK_CURRENT. Chatops never uses ROLLBACK_CURRENT since chatops always wants to check with the latest deployment on the environment.