Update redis-client to v0.21.1
What does this MR do and why?
This fixes a regression in v0.21.0 when no Sentinel masters can be resolved: https://github.com/redis-rb/redis-client/pull/183
Edited by Stan Hu
This fixes a regression in v0.21.0 when no Sentinel masters can be resolved: https://github.com/redis-rb/redis-client/pull/183